Thank you for using the ADTRAN Forums, but you have what sounds like a very unusual problem.
Nothing in what you describe should cause a reboot. It sounds like there is a voltage or grounding issue with one of the modules. It's possible the FXS module may have a short
Does the ATLAS reboot if you call from an FXS port to a local number? Can you call a long distance number if you connect your phone to the second FXS module? Can you move the FXS module to another slot (and modify the configuration to match) and place a call out to see if it reboots when in another slot?
If the problem is isolated to the one FXS module then it will need to be repaired. If the FXS module works in other slots, then it is probably the backplane connector for that slot, and the chassis will need to be repaired.
Hope this helps,