Polycom IP320 phones register, lose registration and will not re-register

I have  4 Polycom IP320 phones and a Linksys ATA that I'm having trouble with.  The Polycom phones will register (to the PBX) and will stay registered.  The Polycom will register, stay registered for a time (less than 5 minutes) and then lose registration.  We are able to make outbound calls from them but not inbound.  Still Googling now and will post if I'm able to resolve it (along with the resolution) but wanted to see if anyone else was having the same problem.  I found one post where someone had had the problem but never saw a response.
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cybertechcafeAuthor Commented:
One other interesting thing that I'm seeing here.  If I reboot one of the phones (looks like any of them), when it registers, the other ones look like they register as well.  They then drop off pretty much the same time.
cybertechcafeAuthor Commented:
Ok, one piece of data that I failed to note earlier.  There are 2 networks (LAN & DMZ) at play here.  The PBX is on the DMZ and the phones are on the LAN (both the Polycom and Linksys).  I turned on SIP debugging (sip set debug on) and saw a lot of traffic.  Short story, I setup a rule on my firewall to allow SIP traffic (UDP 5060) between the PBX and the LAN and the phones have been registered since.
feptiasChief DudeCommented:
This sounds familiar. It is almost certainly linked to the way that firewalls track sessions and will delete sessions from their state table if there is no network traffic between source and destination for longer than X seconds. Typical value for X is 90 seconds. The behaviour varies from one make of firewall to another. Most IP phones and most PBX's can be configured to send keep-alive packets intended to overcome this problem.

Setting up a port forwarding rule (or just a rule to allow SIP udp 5060 traffic) on the firewall would override the default behaviour and allow packets through at any time.

You probably knew all this anyway, but that is what it sounds like to me.

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I agree with feptias; cybertech, what is the model# (and if you know it the firmware version) of the router you utilize ?
cybertechcafeAuthor Commented:
The issue was indeed with the firewall.  I opened SIP (UDP 5060) from the phones to the PBX through the firewall and all seems well.  Phones have been working since.  The one thing that still has me puzzled though is why the Linksys ATA (rebranded Sipura) was able to maintain registration when the Polycom phones were not.  Doing a sip show peers and comparing the two did not show anything and the extensions were setup identical (generic SIP device, etc.).  I also tried changing the nat= settings (yes | no | never | route) to no avail.
feptiasChief DudeCommented:
It could be the setting on the ATA that tells it to ping the Asterisk server with "keep-alive" requests. Linksys phones and ATA's have a setting "NAT Keep Alive Enable" - it is in the NAT Settings section of the Line 1 tab on the SPA3102. When this is set to "yes", it will send a SIP NOTIFY (or you can change it to OPTIONS) request to the SIP server every 30 seconds or so.

Another element of device-specific behaviour that could make a difference is the frequency with which SIP requests are sent to the server (keep-alives or repeat registration requests). If the device only sends requests occassionally, then the firewall may regard each request as a new NAT session rather than a continuation of the old one - it then assigns a new external port on the WAN/DMZ interface and drops the old session. If the server fails to update its contact information with the new port number then it will not be able to send requests to the UA device through the firewall.
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