I configured H.323 Gateway between CME 10.5 and CUCM 12, unfortunately I have the following challenges with calls:
CUCM to CUCM = Successful
CME to CME = Successful
CUCM to CME = Successful
CME to CUCM = Unsuccessful. for this, If I call from CME to CUCM, the phone registered under CUCM rings and when I answer the call, the voice is not passing through (If I answer call from CME to CUCM, there is no voice and calling phone i.e. phone from CME gives a busy tone after some 10 seconds. Note that If I answer call from CME to CUCM, the phone from CME still giving a tone that indicates the outgoing call then after 10 seconds it gives a busy tone).
Any documentation regarding this will highly be appreciated.
Then it sounds to me like you have a problem with session progress messages being negotiated when CME calls CUCM. The CME is not receiving the H225 Connect message sent by CUCM when the call is picked up on the CUCM side, or CUCM is expecting H245 negotiation at call pickup which the router is not responding to. I'm thinking this is a mismatch between H323 Fast Start (on the router) and Slow Start (on CUCM).
On the CUCM H323 Gateway configuration page, in the "Call Routing Information - Inbound Calls" section, look for a checkbox "Enable Inbound FastStart". If that is not checked, check the box and reset the gateway and see if that fixes the problem.
If that doesn't work, we'd need to look at your config.
(But I'd still plan on converting to SIP at some point. It is your best long-term plan.)