The letter by FCC Wireless Telecommunications Bureau Chief John B. Muleta is the first time the commission has addressed, according to Verizon Wireless spokesman Jeffrey Nelson.
Muleta was responding concerns that Verizon raised two weeks ago when it retreated from a once-vehement opposition to the FCC'srequirement going into effect Nov. 24. Though Verizon said it wouldn't fight any customer requests to keep phone numbers, the company believes competitors would throw roadblocks in the way of some deadbeat subscribers.
"Carriers cannot refuse to (transfer the number) while attempting to collect fees, or settle an account," Muleta wrote.
"Today, consumers who wish to change service providers may request service from a new carrier at any time, regardless of their standing with their old provider," he continued. "Consumers must have the same freedom in a number portability environment."
Nelson called the letter "a grand slam for Verizon." There was no immediate reaction Friday from other carriers.
AT&T Wireless, Cingular Wireless, T-Mobile USA and Nextel Communications all say they will comply eventually with the number-portability mandate.
Landline carriers began letting defecting customers keep old telephone numbers in 1999.