I have a customer with a DS2000 and recently migrated away from Verizon to Comcast. The system has worked well for 10+ years. Comcast analog trunks will fail 1 out of 5 calls, more less after many many test calls. Ordinarily we get caller id on incoming calls. For the calls that fail, we do not get caller id and the call drops after 2 rings.

I turned off caller id for all 4 lines and the calls ring in normally with no issue. I re-attached a Verizon trunk and the calls worked perfectly every time.

I am inclined to believe there is some adjustment in the NEC to make or we have a flakey EMTA from Comcast.
The back and forth game is that it works with the buttset so it must be the PBX. I see in program 0402 there is a caller id detection timer that may require adjusting? Everything is at default for the most part.