The only thing I can think to try is to turn on SIP messeges.
On your NRS when the SIP phone is attempting to register turn on the following messages from the vxshell:
syslogLevelSet tSPReg,7
syslogLevelSet tSPReg,6 sets the messaging back to info from debug.
These message might show you why it is failing, or even if it is sending a registration request.
If the you have the SIP phone configured for static in the NRS turn on the following messge in the vxshell on you cs1k leader:
SipNpmDebugPrint=1
SipNpmDebugPrint=0 disables the messaging.
This will how if the invite is being re-directed by the NRS, and if so the response received back from the SIP phone.
Low traffic periods are best for the SipNpmDebugPrint command.