Hello @markf100
If you setup a trunk next to the general tab there is a tab for you to add DID's. You can then direct these DID's via the inbound rules.
There is also IVR capabilities built in the system.
Please go the academy slides that explain most of the functions of the PBX and how they work https://www.3cx.com/3cxacademy/
Also the admin manual contains a lot of crucial info regarding the operation of the system https://www.3cx.com/docs/manual/