You can contact your carrier and tell them , providing the both spans are under the same carrier . The can set up failover and test it out . Been there many times !!!
If you have 2 spans :
1. channel 23 should be busy and send traffic over to channel 25 ( or channel 1 on the 2nd span ) .
2. If you unplug span one traffic should revert over to span 2 . Carrier issue ...
Thanks for the input. Actually I am the carrier.
I opened a TT with Adtran and the response I got was as follows.
A 503 is the expected response on a full trunk. There are known issues in AOS R10.9.4 that can result in the behavior you're seeing. Please upgrade to R10.9.8 or R11.4.3 and retest while running the debugs from my previous email.
ADTRAN Technical Support
We will upgrade as suggested and repost the results.