When you make that change from analog gateway to digital (e1), all your troubles with signaling and answer supervision should go away like magic.
All those signals will go now in a diferent channel on the E1 betwen the gateway and the cental office equipment on the phone company.
Like a independent data link betwen the brains of the gateway and the brains of the phone company equipment.
The answer supervision will work so much to the second, that even after several months, the records on one end of the circuit, and the records on the other end will match to the second.
If you could do to E1 interface, that will be the best think that you could do.