This is a snippet from the article Metaswitch has posted on their Communities site regarding best configuration practices for the Adtran TA900 and NetVanta products.
Note: For the TA900 series, if the party answering the registration requests from the device (typically the CFS or a SBC like perimeta) sets an expiration time below the default registration threshold of 300s the result is the 5 second registration renewal. To avoid this you will need to modify the registration threshold to a number that is less that the expire time given from the registrar. For example, you could add the following to your SIP trunk configuration:
(config)# voice trunk T01 type sip
(config-T01)# registrar threshold absolute 20
This command tells the unit to renew the registration 20 seconds prior to the expiration. .
I think I may have resolved my issue. I discovered that I had my SIP (FXS) test line in the MetaSwitch set to expire every 10 seconds (a setting I forgot to undo from another test), which is below the TA908e's default registration threshold of 300 seconds. I believe that was causing the 5 second registration renewal issue. I changed the expire timer to 3600 seconds, and so far the 5 second registration requests have stopped.
I used the above article MetaSwitch Communities article to set everything up, but did not fully understand that note and realize what was happening until now. Thank you for the reply!