A IP telephony service allows customers to form user groups. Each user group can include multiple members, each of whom have their own telephony device. A group identifier, which could be a telephone number, is assigned to each user group. As a result, a single telephony device belonging to a member of a user group can be associated with its own unique telephone number, and the telephone number used as the group identifier. The system allows the user to select which of multiple telephone numbers associated with his telephony device are to be used as the originating telephone number when an outgoing telephone call or an outgoing message is sent.