You dont say whether you have a straight T1 or PRI circuit(s). If you check your DTA and/or DTI errors, you may be able to find out exactly what is causing your problems, so that should be 1st on your TODO list after bringing the circuit back up.
As GHTROUT stated, most often, issues like this are CO related (maint after hours), due to your local telco running some sort of maintenance (and they will never tell you).
If you have PRI circuits with D-Channels: go to LD 96 and stat your DCH's. The response should look something like this:
>ld 96
DCH000
.stat dch
DCH 004 : OPER EST ACTV AUTO DES : OPT_61C
Check that your DCH setting is for AUTO. That way, once the D-Channel re-establishes, the DCH and hopefully your loop will auto recover.
ENL AUTO x (DCH)
Keep in mind, the problem 'could' be telco, but it could also be clocking or hardware on your side. It wouldnt hurt to go into LD 60 and type LCNT to see if you are getting clocking errors. Sometime the circuits will shut down when a certain criteria has been exceeded.