Certain Mobile number cannot be called

[Gfurr]

Every time I call a specific number through 3CX it always comes up as busy (this was not a problem in the past).

However, if I call with another device like my mobile I am able to get through to that mobile every time.

 

[Alphabetic]

Check the call logs, it may be that your mobile app i sending calls in the E164 format and the other endpoint isnt.

check the outbound rules too.