Is it possible to set up 3CX to route inbound calls directly to the echo service (*777) so that the call is
answered and one can test two-way voice from e.g. a mobile phone calling to a DID?
You could set an inbound route to hit a ring group with 5 sec timeout then enter in the forward to external number and enter *777 as the external number, a bit of a workaround but it works.