Hi Michael,
I can help you with the configuration of cisco 7960 (SIP image) with hosted SIP provider. Most of the time it is because of NAT firewall, and the success will depend on the type of NAT firewall also.
You should have a working TFTP to store config file, MAC address of Cisco, and SIP credentials of provider as well.
I can start now via Teamviewer.
Thanks,
Bao