Agree with above. If this is a standard Robbed Bit Signaling T1, almost every CO switch out there can divide it into 3 different trunk groups (presumably 1-8, 9-16, 17-24), and assign a different outbound caller id to each trunk group. Then, on inbound, the numbers would be pointed to each trunk group. In the PBX, you would set it up so that each company accessed the proper set of channels for outbound, and that the phone numbers coming in on each set of channels went to the proper place. If it's a PRI, then you the PBX can send the caller ID desired, and it should work fine.