We recently purchased a SIP trunk from an ITSP, our CUCM (cluster of two CUCMs) is behind corp firewall, ITSP only establishes this SIP trunk with one public IP from us, so I configured static mapping on firewall to map our public IP to CUCM-SUB and allow SIP traffic to come in.
Outbound call signaling is fine, i.e, calls from internal phone to PSTN phone is routed correctly by this SIP trunk, but there is no audio, further debugging shows that CUCM-PUB is trying to initiate RTP streams to ITSP's SIP gateway, this is not going to work because source from CUCM-PUB will be PATed to a totally different public IP, and ITSP's SIP gateway won't accept the connection. I am wondering how I can force CUCM cluster to always use CUCM-SUB to establish SIP trunk and terminate audio? or I can get away with the problem by doing something differently on firewall? if firewall can be configured to mapping two internal IPs to one public IP?