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?

  • Do these issues persist if you try to use a phone at a 3rd site (that is, not the two you mention above)? – Driftpeasant Dec 2 '11 at 3:12
  • Have you tried eliminating the router entirely? – Joel E Salas Dec 2 '11 at 4:01
  • I have not tried eliminating the router entirely because the mean time between failure events can be several days. And I need that internet connection to do work. I do not think a 3rd site would add new information, because the router is the only device separating the voip phone from the PBX. – steampowered Dec 2 '11 at 4:11
  • A third site would tell you if it's that specific router or if it's the endpoint (or possibly the remote site internet connection). I have a client site with VOIP phones in a similar configuration. We have Cisco ASAs at either end, so we know the firewall/router settings are correct. Calls still drop, however. We traced it to what the vendor calls "packet jitter" - if the average latency between the two sites goes over 40ms for any one call, the call drops. It's not a router problem there - it's an internet connection one. We ended up having to put a 2nd phone server at that location. – Driftpeasant Dec 2 '11 at 14:06

The problem had to do with bandwidth available to the voip phone. Increasing the bandwidth of the remote internet connection to 50 Mbps down/ 12 Mbps up caused the problem to go away.

I suspect I could have made it work with less bandwidth if I had properly implemented QoS at the remote internet connection to prioritize SIP traffic. I did set up QoS at 50 Mbps, but I never lowered the bandwidth during testing to determine if QoS alone would have solved the problem.

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