This scenario took me quite a while and still no luck.
One co-resident CME-A gateway/ H.323 gatekeeper in same cisco rtr, and one other CME-B gateway.
ephones under CME-A cannot connect to CME-B via gatekeeper, CME-B ephone cannot talk to A vise versa,
Each time, after dial digits, got busy signal.
Both CME-A and CME-B registered to Gatekeeper. All of zone prefixs are also up.
If without Gatekeeper, using IPIP gateway config, both CME-A and CME-B ephones can communicate.
Routing between CME-A ephone and CME-B ephone been verified.
The issue only happen when calls been routed through gatekeeper.
I am afraid it is due to co-resident CME-GW/GK issue.
The config can be provided if needed.