Google’s Voice calling application is adding a long-promised feature: the ability to move a phone number from a cell phone to Google’s service.

Previously, Google Voice assigned each user a new number. That made it more difficult to take advantage of its features, which include cheaper international calls and the ability to answer calls on several phones and computers.

Google Inc. announced Tuesday that users can request “porting” of existing numbers to Google Voice. The transfer costs $20. Service would be terminated for the cell phone, unless the user requests a new number for that line from the phone company.

A leading competitor, Skype SA and its Internet calling service, does not offer number porting.