Although the Federal Communications Commission has set a target of 2.5 hours for carriers to complete the transfer of a number to a rival, experts and most of the companies say the process is likely to take at least a day at first.

"We'll be a heck of a lot better at this on Dec. 30 than on Nov. 30," said Howard Waterman, a Verizon spokesman.

Martin Dunsby, an analyst with the consulting firm inCode Telecom Group, said that it could take weeks before the process is "seamless and speedy" and that problems will persist much longer.

Other glitches may depend on whether carriers intentionally impede the process when they lose a customer.

"Some carriers will be quite particular with information," said Sprint spokeswoman Jennifer Walsh. For example, she said, a carrier may hold up the process if customer abbreviates a word in his address such as "street" as "St" while it appears as "St." on the old carrier's bill.

Many people lured to new carriers by sweet promotions and rosy promises about service may be disappointed once their switch goes through.

In fact, Dunsby predicts a reverse flood of requests by customers wanting to move their numbers back to their old service.

"In six weeks, we may see a big un-port," he said. "I think it'll be a grass-wasn't-greener effect."