There are products out there that can do the interworking between H.323 and SIP (the Cisco Expressway products do this) - also products such as Pexip can do this, as well as providing a gateway to the MST environment.
The other alternative is to reconfigure your endpoints as SIP endpoints rather than leave them on H.323, unless there is some specific reason (due to them being ancient, or configured as standalone units, or something similar) that you can't do that. Most endpoints released in the last 10 years have been able to be SIP registered.
Wayne
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