This may be related to NAT on the router,
enable sip-alg on your router (may require a firmware update)
does it work plugged into the cable modem
or possibly forward ports to the phone through the router
sip is usually 5060-5080, (admin-->advanced-->ext x - sip settings --> sip port)
RTP on these devices is usually
16384 to 16538
I think verify at admin-->advanced-->sip - RTP parameters --> rtp port min/max
Your voip account provider may have a sample config that will work with your router, and a cisco SPA phone. The model shouldn't matter, the network settings are very similar between devices.
or call into the small business TAC. Easiest way is go to
cisco.com/smb --> support open a case, then once you have your case # call in 866-6060-1866 Listen to the prompts, pick your language, use "existing case" opt 2 I think
Hope it helps, please rate helpful posts,
Dan