I'm assuming that you're referring to analog FXS ports, if not please advise.
Is it possible that some of the connected devices are configured for pulse dialing? The TA908 series doesn't support it, DTMF only.
If you are still looking for an answer to this, I had the same issue on TA 924s running 12.3.4 firmware. I downgraded mine to R12.3.1.E and the issue never occurred again. At the time of downgrading, Adtran support had stated the issue was present in 12.3.4 and would be fixed in a future release to it is possibly fixed by now.