A system and method for providing user mobility handling in a network telephony system is provided. In one embodiment, the method includes receiving a register request from a network entity on a first domain and modifying a user identifier in the register request. A registration database is preferably updated with the modified user identifier. The modified register request is transmitted via a network to a network entity on a second domain. An authentication process preferably takes place between the network entity on the first domain and the network entity on the second domain. If successful, a confirmation is received via the network from the network entity on the second domain. Various embodiments of the invention may, for example, allow a user typically located in a home domain to temporarily move to a visiting domain, while maintaining the user's home domain user identifier. Preferred embodiments utilize the Session Initiation Protocol.