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Hi

I would generally use rfc2833, which you set in the trunk.

However be aware that not all carriers allow reverse DTMF. What I mean by that is DTMF coming from the called party, rather than the more usual DTMF going to the called party.

Before you regard this as the problem, ring an IVR via your trunk you use for callback, and check the the DTMF is being passed to the called party.

Joe