So many possible causes.
At the first, capture SIP packets to verify the phone is trying to register to proper SIP proxy. If yes, upstreab provider shoudl be able to explain why your REGISTER has been rejected. But captured answer to REGISTER may disclose the true cause to you with no upstream help.. Especially if you will have REGISTER/response from both working and non working device, so we can compare them.
Syslog&debug messages (preferably from both devices) may help as well.
Debug and syslog Messages from the SPA3xx, SPA5xxG, SPA9xx, & WIP310 IP Phones
As a blind shot - configuration is not the same. Or - both configurations are same and your upstream doesn't allow simultaneous registration of two devices using same credentials. While "simultaneous" mean registration of other device before registration of the first device has expired.