Usually the SIP device cannot distinguish (reliably) between PBX-internal and PBX-from-external calls though, and will treat both as "external" (in the meaning of - call comes from outside the DECT device).
The snom 3xx and 8xx wired phones allow for additional SIP headers in call setup to cause different ringtones. AFAIK the m9 does not support this, sorry.
How can i set Distinctive ringing for the 3cx phone, there is no template for 3cx phone ?
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.