This was happening to me (along with a few other strange issues) for a month now. I finally got around to opening a ticket this morning and I was given two solutions:

1) try opening this particular range of ports in replacement of these older rules:

- SIP Command Ports 5060-5080 TCP
- RTP Audio/Command Ports 5004-65000 UDP

2) try placing the RT31P2 upstream to your D-Link router to place it in front of your firewall.

I just went with the second recommendation and put the VOIPo router upstream from my main router. The path is now Cable modem ---> RT31P2 ----> DIR-615. I tried a couple of test calls from my cell and so far so good.