E&M wink RBS signaling is symmetrical. You should be able to bookend two TA900 units set for RBS wink role network to each other and have it work just fine, or one to a PBX, or one to a telco wink-start T1 RBS trunk. The word "network" in an RBS configuration is just a required keyword that really does nothing.
Specifically how is the trunk not performing as expected?
The only IP Business Gateway AOS device that supports Telco/PSTN facing CAS trunks is the NV 644 - it can do both USER and NETWORK role.
All other IPBG AOS devices only support NETWORK role for CAS trunks. The intended design is for the IPBG to act as the NETWORK for a PBX. Other than the NV 644, they are not intended to terminate PSTN CAS trunks or act as the USER for a PBX configured as NETWORK.