Hi
You might play with translation-rules in your H323
You might use an incoming dial-peer that matches CALLING PARTY NUMBER using answer-address command.
Then something like..
voice translation-rule 1 Assuming we only receive 1200 as called number..(1200 is our extension).
rule 1 reject /^1200/
voice translation-profile profile1
translate called 1
dial-peer voice 1000 pots
answer-address 9788587088 <--- Calling number
translation-profile outgoing profile1
direct-inward-dial
port 1/0:23
http://www.cisco.com/warp/public/788/voip/voice_transla_rules.html#reject