The Smart Phone Control Protocol, or SPCS, is an addition to the Skinny Call Control Protocol (SCCP) that is only supported on the CP-500, SPA500, and SPA525G and SPA525G2 phones.
(Note CP500 only support SPCP whereas SPA5xx support SPCS or SIP)
79XX phones register with their SCCP call control using a proprietary handshake that consists of a series of messages. During the registration phase a number of things happen:
1. Phone (Station) sends a Registration Request
2. Request is acknowledged by the Call Control (CME/UC500)
3. Capabilities are advertised
4. Button template, Softkeys, Line Status, Display message, Speed dials Time and date are requested by the phone and sent by CME/UC500
When the phone is a 500 series phone, two additional messages precede all this handshake, namely an SPCP Token Request and an SPCP Token Acknowledgement (if successful) or Token rejection (if unsuccessful). A Token ACK can only be sent by UC500, while an ISR will always send a Token Reject. This prevents the phone from registering to any call control different than UC500.
We have seen a case where if you build a SPA525G phone as a TYPE SPA525G2 phone in the UC500, it will give you an SPCP Token Error when it tries to register. You would see this on the console port or the Telnet session with 'terminal monitor' enabled.