These are circuits that are between an Adtran TA900 and the customer's PBX that replaced T1s that were once point to point T1s back into another carrier's 5ESS. It took some time to figure out this combination because the original phone vendor was no longer around and it took trial and error to discover this because the circuits would take errors and slips.
There is no reason why this would not work but there is no reason to do it unless there is some limitations on old equipment. Most of the PBX's that were installed with this configuration were put into service in the early 90's when PRIs and PRI cards were considerably more expensive than T1's. None of these PBX's can utilize Caller ID name even with a PRI and only a few of them will take Caller ID number using Feature Group D.
I do not, have not, and will not work for AT&T/Comcast/Verizon/Charter or similar sized company.