My Aastra 6757i off-site voip phone locks up periodically, and I want to rule out a networking problem. Most of the time calls work fine, both placed and received calls. But sometimes the voip call freezes and the phone reboots. I have tried multiple identical voip phones. The phones inside the LAN never lock up, but phones moved outside the LAN in this configuration all lock up periodically.
The Aastra 6757i voip phone connects through the internet to a Zultys MX250 PBX in another US state. The MX250 is connected directly to the internet using its own static public IP. No firewall separates the MX 250 PBX from the internet except the specialized PBX software firewall (in the PBX and part of the PBX software).
The internet connection is good at both ends. Remote site has 20 Mbps down/ 5 Mbps up over a Comcast cable modem, and the PBX site has 3 Mbps down/ 3 Mbps up over a dedicated, bonded T1.
The Aastra voip phone is separated from the internet by a D-Link DIR-625 higher-end home router. The DIR-625 is the only device in between the off-site voip phone and the PBX.
The DIR-625 has SPI disabled, NAT endpoint filtering set to endpoint independant, TCP endpoint filtering set to endpoint independant, anti-spoof checking disabled, ALG disabled for SIP.
I recently put the phone on the home router's DMZ, but this change did not have any noticable effect.
Assuming the PBX is set up correctly, what can I do at my remote location to help my voip phone connect to the PBX? Are my home router's firewall settings correct?