Direct call to number#extension - how to?

[StefanW]

in one way you can not dial this, as the number you call 12345#EXT is not a number and overlap dialing is something from the past.

You can point a DID (12345) to an IVR (not 999) and once it announces something dial the ext number. You will connected.
Check this video: https://www.3cx.com/3cxacademy/videos/intermediate/digital-receptionist/

 

[leejor]

You are going to have to get a DID number/service. They exist to do what you are asking for.