Hello Atinc,
M3UA is supported by PGW, not M2UA.
I briefly checked 7515 MG. It looks like it's a Media Gateway and it soesn't have STP functionality. So it doesn't support M3UA/M2UA.
It uses MEGACO protocol and it's controlled by Call Agent.
MEGACO is supported on PGW 9.8(1) to control some cisco gateways.
When you add a gateway you have to speify ext-node type. And ext-node describes which kind of gateway/device you have. As 7515 is not there I doubt it will be possible to add this type of gateway.And it's definitely not the best practice.
Best regards,
Alexei Piskunov.
