"This was one of the most telecom-intensive areas of the world," Larry Babbio, president of New York-based Verizon, told reporters.
Two Verizon offices situated near the World Trade Centre (WTC) took heavy blows - as WTC's "Building 7" toppled, beams speared a Verizon nerve centre that served some 200,000 lines, said Babbio.
Much of the Verizon equipment at the company's lower Manhattan address on West St, the centre nearest to the WTC, is housed in a basement and badly damaged with water and soot.
"If you cover your computer with dirt and pour water on it, it is not going to operate for very long," said Babbio.
"The work we have to do to that office is enormous," he continued. "That office, along with our facility on Broad St, delivers primary service to the New York Stock Exchange."
Babbio estimated that 80% of New York Stock Exchange telecom services are serviced by the Broad St facility, although 20% of the private line services came through the crippled West St facility.
Verizon reported that the cellular call volume was 50 to 100% above normal and had put in seven replacement sites. One Verizon cellular site at WTC was destroyed.
Almost all 500 Verizon employees working in WTC had been accounted for, though some had almost certainly perished, the emotional Verizon chief said.
AT&T Wireless reported losing access to some network sites near the WTC on Tuesday. AT&T, AT&T Wireless and Sprint said an inundation of calls was taxing their networks, but calls are still getting through.
AT&T Wireless gave 1,300 mobile phones to local law enforcement and rescue organisations to assist with immediate communication needs.
The loss of some network equipment that was housed at the WTC has forced Sprint to reroute some calls to other facilities.
"We had some network equipment in one of the twin towers and obviously, it was destroyed," said a spokesman.
AT&T said in a statement it had suffered no damage to long-distance networks as a result of the terrorist attacks, but said disruptions to service were the result of high call volumes. The company requested that unnecessary calls should not be placed to New York or Washington.
AT&T Wireless reported one of its heaviest call volume days ever, but the network is functioning overall. The company had a handful of sites down that were in or near the Trade Centre.
The company requested that customers avoid making non-essential calls into and out of critical areas on the East Coast so that more circuits are available for emergency use.
"We're asking customers and employees to try to refrain from using their wireless phones unless it's urgent," said AT&T Wireless.
To further improve coverage in the city, AT&T Wireless is rerouting traffic and redirecting some of its antennas.
It has deployed "COWS," or Cells On Wheels (mobile cellular towers), in Jersey City and Brooklyn to help improve call capacity.