Maybe there is a compatiblity issue but I am not sure... Some limitations on chipset itself... You can confirm with TP-LINK. http://ticket.tp-link.com/index.php?/Tickets/Submit/RenderForm

Did you login http://192.168.1.1? Go to Interface Setup---LAN and you can change the IP address.

Access Point: 1. Login 192.168.1.1 page. Interface Setup---LAN. Change IP address to be the same subnet as the root router. Eg. if Router‘s IP is 192.168.0.1, change W8951's IP to 192.168.0.2. Save...

Did you confirm it with your VoIP provider? It doesn't seem like a configuration issue on modem itself.

Did you configure Advanced Settings? Do they match each other on both sites? How about typing IP address in Remote IPSec Gateway Address? And I noticed that the firmware is not the latest version.

Follow this to troubleshoot. http://www.tp-link.com/en/article/?faqid=673 If it won't help, get it replaced under warranty.