Core Issue
Cisco Session Initiation Protocol (SIP) IP phones are full-featured telephones that can be plugged directly into an IP network and can be used very much like a standard PBX telephone. The Cisco SIP IP phone is an IP telephony instrument that can be used in VoIP networks.
The Cisco SIP IP phone terminals can attach to the existing data network infrastructure via 10BASE-T/100BASE-T interfaces on an Ethernet switch. When used with a voice-capable Ethernet switch (one that understands type of service [ToS] bits and can prioritize VoIP traffic), the phones eliminate the need for a traditional proprietary telephone set and key system and private branch exchange (PBX).
The Cisco SIP IP phone complies with RFC 3261, as described in Information About SIP Compliance with RFC 3261.
Resolution
This feature is currently not supported in the SIP IP phones.
