Does anyone know any IP Phones on the market that can do a SIP account login on a dynamic basis? IE in a call centre enviroment where a phone may be used by more than one person throughout the day but we would want each person to have their own unique extention.

Ideally something with a 'Login' button that would then prompt for extenion number and then password. I'm note sure such thing exists but if anyone know of something that would do the job please let me know.