Cisco Unified SIP Phone 3900 Series support Session Initiation Protocol (SIP). The series includes of 3905, 3911(End-of-Sale model) and 3951(End-of-Sale model). These SIP Phones offer 10/100 integrated switches, single line of key, full-dulpex speakerphone and 8 fixed feature keys. The 3900 Series can be placed on a desktop or wall-mounted.