- Malaysia Airlines says there's no evidence of phone contact from Flight 370
- Telecom experts say plane may have been flying too high or fast for passengers to make calls
- "Technically it is possible" that cell towers in Malaysia or Thailand could have registered a signal from phones
(CNN) -- It's a popular question on social media: Why didn't passengers on board the missing Malaysia Airlines Flight 370 make mobile calls?
Many recall that when United Flight 93 was hijacked on September 11, 2001, passengers were able to make two cell phone calls during the flight's final moments. Several other calls were made using airphones.
If metadata was detected from cell phones on Flight 370, surely it would shed more light on the missing plane's flight path?
The plane may have been flying too high or too fast to register with cell towers, according to telecoms experts, but careful analysis of the passengers' cell phone records will need to be completed to be certain.
A distraught relative of a passenger on the missing Malaysia Airlines jet breaks down while talking to reporters at Kuala Lumpur International Airport in Sepang, Malaysia, on Wednesday, March 19. The Boeing 777-200ER disappeared during a March 8 flight from Kuala Lumpur to Beijing. Searchers from 26 countries are trying to pinpoint Flight 370's location somewhere along two vast arcs, one stretching deep into the Asian landmass, the other far out into the Indian Ocean. 
A relative of Chinese passengers aboard the missing plane waits for a news briefing by officials in Beijing on Tuesday, March 18. 
A relative of a Chinese passenger aboard Malaysia Airlines Flight 370 tells reporters in Beijing on March 18 about a hunger strike to protest authorities' handling of information about the missing jet. 
A member of Indonesia's National Search and Rescue Agency joins in a search for the missing plane in the Andaman Sea area around the northern tip of Indonesia's Sumatra on Monday, March 17. 
Relatives of passengers aboard Malaysia Airlines Flight 370 watch a news program about the missing plane as they await information at a hotel ballroom in Beijing on March 17. 
Malaysian Transportation Minister Hishamuddin Hussein, center, shows maps of the search area March 17 at a hotel in Sepang, Malaysia, next to the Kuala Lumpur International Airport. 
U.S. Navy crew members assist in search-and-rescue operations Sunday, March 16, in the Indian Ocean. 
Indonesian personnel watch over high seas during a search operation in the Andaman Sea on Saturday, March 15. 
A foam plane, which has personalized messages for the missing flight's passengers, is seen at a viewing gallery March 15 at Kuala Lumpur International Airport. 
A member of the Malaysian navy makes a call as his ship approaches a Chinese Coast Guard ship in the South China Sea on March 15. 
A Indonesian ship heads to the Andaman Sea during a search operation near the tip of Sumatra, Indonesia, on March 15. 
Elementary school students pray for the missing passengers during class in Medan, Indonesia, on March 15. 
Col. Vu Duc Long of the Vietnam air force fields reporters' questions at an air base in Ho Chi Minh City, Vietnam, after a search operation on Friday, March 14. 
Members of the Chinese navy continue search operations on Thursday, March 13. The search area for Flight 370 has grown wider. After starting in the sea between Malaysia and Vietnam, the plane's last confirmed location, efforts are expanding west into the Indian Ocean. 
A Vietnamese military official looks out an aircraft window during search operations March 13. 
Malaysian air force members look for debris on March 13 near Kuala Lumpur. 
A relative of a missing passenger watches TV at a Beijing hotel as she waits for the latest news March 13. 
A member of the Indonesian National Search and Rescue Agency scans the horizon in the Strait of Malacca on Wednesday, March 12. 
Relatives of missing passengers wait for the latest news at a hotel in Beijing on March 12. 
Journalists raise their hands to ask questions during a news conference in Sepang on March 12. 
Indonesian air force officers in Medan, Indonesia, examine a map of the Strait of Malacca on March 12. 
A member of the Vietnamese air force checks a map while searching for the missing plane on Tuesday, March 11. 
Iranians Pouri Nourmohammadi, second left, and Delavar Seyed Mohammad Reza, far right, were identified by Interpol as the two men who used stolen passports to board the flight. But there's no evidence to suggest either was connected to any terrorist organizations, according to Malaysian investigators. Malaysian police believe Nourmohammadi was trying to emigrate to Germany using the stolen Austrian passport. 
An Indonesian navy crew member scans an area of the South China Sea bordering Indonesia, Malaysia and Thailand on Monday, March 10. 
Vietnam air force Col. Le Huu Hanh is reflected on the navigation control panel of a plane that is part of the search operation over the South China Sea on March 10. 
Relatives of the missing flight's passengers wait in a Beijing hotel room on March 10. 
A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews before returning to search for the missing plane Sunday, March 9, in the Gulf of Thailand. 
Members of the Fo Guang Shan rescue team offer a special prayer March 9 at Kuala Lumpur International Airport. 
A handout picture provided by the Malaysian Maritime Enforcement Agency shows personnel checking a radar screen during search-and-rescue operations March 9. 
Italian tourist Luigi Maraldi, who reported his passport stolen in August, shows his current passport during a news conference at a police station in Phuket island, Thailand, on March 9. Two passengers on the missing Malaysia Airlines flight were reportedly traveling on stolen passports belonging to Maraldi and an Austrian citizen whose papers were stolen two years ago. 
Hugh Dunleavy, commercial director of Malaysia Airlines, speaks to journalists March 9 at a Beijing hotel where relatives and friends of the missing flight's passengers are staying. 
Vietnamese air force crew stand in front of a plane at Tan Son Nhat airport in Ho Chi Minh City on March 9 before heading out to the area between Vietnam and Malaysia where the airliner vanished. 
Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9. 
The Chinese navy warship Jinggangshan prepares to leave Zhanjiang Port early on March 9 to assist in search-and-rescue operations for the missing Malaysia Airlines flight. The Jinggangshan, an amphibious landing ship, is loaded with lifesaving equipment, underwater detection devices and supplies of oil, water and food. 
Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9. The vessel is carrying 12 divers and will rendezvous with another rescue vessel on its way to the area where contact was lost with Malaysia Airlines Flight 370. 
The rescue vessel sets out from Sanya in the South China Sea. 
A family member of missing passengers is mobbed by journalists at Kuala Lumpur International Airport on Saturday, March 8. 
A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported March 8. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10. 
Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8. 
Malaysia Airlines official Joshua Law Kok Hwa, center, speaks to reporters in Beijing on March 8. 
A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8. 
Wang Yue, director of marketing of Malaysia Airlines in China, reads a company statement during a news conference at the Metro Park Lido Hotel in Beijing on March 8. 
Chinese police at the Beijing airport stand beside the arrival board showing delayed Flight 370 in red on March 8. 
A woman asks a staff member at the Beijing airport for more information on the missing flight. 
A Malaysian man who says he has relatives on board the missing plane talks to journalists at the Beijing airport on March 8. 
Passengers walk past a Malaysia Airlines sign on March 8 at Kuala Lumpur International Airport. 
Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference on March 8 at a hotel in Sepang. "We deeply regret that we have lost all contacts" with the jet, he said. The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
The search for Malaysia Airlines Flight 370
Photos: The search for Malaysia Airlines Flight 370 
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See officials remove screaming mothers "So far, we have not had any evidence of any telephone company of any member trying to contact," said Malaysia Airlines CEO Ahmad Jauhari Yahya at a news conference on Monday.
"But anyway, we are still checking. There are millions of records to process. It is being done as part of the investigation."
Was the plane too high?
According to radar analysis, the plane is believed to have been flying as high as 45,000 feet and as low as 23,000 feet.
But even this lower altitude is too high to register with mobile towers, experts say.
"If you look at the data in this case, the altitude at which the planes were traveling is too high," Vincent Lau, wireless communications specialist and professor at the Hong Kong University of Science and Technology's Department of Electronic and Computer Engineering, told CNN.
"Even on the ground it wouldn't be easy to pick up from that distance, and if you are flying it's even more difficult because at those angles you are only picking up what we call leakage from the side loops of the antennas, which are substantially weaker than the signals from the main loops of the base stations."
While business class seats on the aircraft are known to have been equipped with phones that worked via satellite, it would be easy to strike down that system from inside the plane, said Lau.
Reports on Monday that the plane flew as low as 5,000 feet or less over mountainous terrain -- possibly in order to evade radar detection -- haven't been confirmed.
"In terms of the altitude it would have to be no higher than around 10,000 feet. Anything higher ... would be problematic," according to Bill Rojas, director of telecom research at IDC Asia Pacific.
Unlike in urban areas, where cell phone antennae are typically pointed down toward the ground, cell towers in rural areas are up to 30-45 meters high and are often pointed at an angle meant to cover wider distances.
So if you're up in the sky, you can receive the signal as well, Rojas said.
"If the airplane were flying over northern Malaysia or southern Thailand -- basically the rural area -- then it's very possible that a cell tower could register the signal from the phones, assuming they were on," said Rojas.
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Retrieving the data
If smartphones had been on and registered with a cell tower, the records would be relatively easy to retrieve.
"The registrations would typically be logged and depending on the operator they will be kept for hours, days or months," said Rojas.
The telecom expert said that he'd place particular focus on the phone numbers of passengers from Thailand or Malaysia.
"I would assume that the authorities are checking with the mobile operators by comparing known passenger cell numbers to see if there were any pings or attempted or successful network registrations in northern Malaysia or southern Thailand or possibly even Indonesia," said Rojas. "Any passenger who had roaming capabilities or a local Malaysian number -- if the plane were over Malaysia -- could in theory have been registered on the network if their phone was on.
"If a passenger does not have international roaming then their access would be rejected by the network and for a period of time that metadata might be stored by the network."
If the metadata records were erased automatically after a few days or a few hours, could they be retrieved?
"Each mobile operator will have its own guidelines for the duration of metadata storage and would not normally be made public for obvious national security and law enforcement reasons."
Rojas said as far as speed was concerned, the aircraft would need to have been flying at speeds below 250 kilometers per hour (155 mph) in order for passengers to make or receive calls.
Passengers on high-speed trains in Japan and other countries can make calls via 3G networks at speeds of up to 240 kilometers per hour, but cell towers aren't able to register a signal beyond those speeds, Rojas said.
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