The Adtran 900 is behind NAT and registers a SIP Trunk to a public IP. The Register expires every 60 minutes and outbound calls work fine.
Inbound calls only work fine for about 2 minutes after the trunk registers. Once the NAT device clears the session, no other inbound calls are allowed until the session is opened again on the next Register.
This is usually easy to get around with by enabling a keep-alive when using SIP phones. The phone will send a dummy or Options packet every 30 seconds for example to keep the NAT session open.
On the Adtran there is a sip-keep-alive parameter for the trunk but it does not send any keep alives when the trunk is idle.
How does one configure the Adtran when its behind a NAT to keep the NAT session open permanently?
After opening a case with Adtran this was the workaround suggested:
The easiest method would be to adjust the SIP Registration timeout on the SIP trunk from 60 minutes to a lower configured value than the firewall timeout value. This will keep the NAT session open.