Unsupported Relay not triggering through 3CX

[JohnS_3CX]

if I call the intercom with my mobile phone the relay will trigger by pressing 6!

The SDP 3CX sends contains the right type of DTMF, hence it appears to work when 3CX initiates the call.

but if the intercom call my mobile phone it doesn't work!

You will have to look at the SDP the intercom sends when it is the initiator. Comparing these two might reveal why it fails. You may have options in its UI to adjust this, until you get it working.

If possible, I would suggest using G.711U/A (aka PCMU / PCMA) as the top codec, and RFC2833 only as the DTMF type.