Ok, so there must be a translation pattern or route pattern that is configured on the CUCM Call Routing settings, which converts the '0' to the in-house PBX number.
If it's a route pattern, you will have to change the device to which the route pattern is pointing to. Right now, it would be pointing to the in-house PBX connection.
Yes, there is a translation pattern that translates "0" to 64026, the CTI route point for the Attendant Console that our in-house PBX uses.
Is there no way to get an outside operator and still maintain that translation pattern?
Yes, just configure another TP to point to the PSTN, you cannot use 0 as it's already used for the CTI RP, you'll need to use another digit/dialing string.
If you want to use 0 for both, you'll need to use partitions and define who will dial 0 and be routed to AC, and who will be routed to PSTN. They cannot have both.