You can create outbound rules, whereby with a simple prefix you can use any outbound caller ID.
For example:
dial 9 to use the main company phone number
dial 8 to use the extensions DID number
dial 7 to dial the main SALES DID number
For inbound rules, rather than using an extension on a phone to differentiate where the call is coming from you can use a ring group and use that to annouce the origin of the call.
For example:
660 (Existing setup is the CEO extension - New setup Create a CEO ring group forwarding to the single extension number)
661 (Existing setup is the Sales Extension - New setup Crate a SALES ring group forwarding to the single extension number)
662 (Existing setup is the Support Extension - New setup Create a SUPPORT ring group forwarding to the single extension number)
With ring groups, when someone calls into the CEO ring group, you will see on the screen of the phone "CEO:<CALLER ID>"
When someone calls to the SALES ring group you will see "SALES:<CALLER ID>" on the phone screen and "SUPPORT:<CALLER ID> " when someone calls the support ring group.
These are just various examples of what you can do with the 3CX Phone System