You most likely need to reassign the number. Here are some examples where you reassign the number, but still maintain the same call handling.
- let's say you want to add a given number as caller ID to user ext.101

1) number is inventory -> assign as additional number to 101
2) number is currently assigned to a call queue -> you need to have 101 as a member of that call queue. If necessary disable 101 from getting queue calls for that queue.

3) number is currently assigned to a site. -> number will be visible if 101 is allocated to the same site. If 101 is allocated to a different site, reassign the number to the Auto Receptionist, and use a custom rule to route it to the same site.

4) number is currently assigned to an IVR menu -> 2 ways to do it: A) you need to reassign the number to the site(configure site call handling or custom rule to route the number to the same IVR menu), and you need to have 101 be allocated to this site. or the better way imo you need to reassign the number to the Auto-Receptionist. Then use custom rule to forward to the IVR menu.


IN SHORT
it depends on where the number you want to add is currently assigned
and to which site the user extension is allocated (if you are using Multi-Sites)

Thank you so much for the detailed response!

If it helps, currently the number I want to add to an Ext to call out from (my caller ID) is a digital line that just routes to the auto receptionist.

I want to make sure we are speaking the same language. Digital line means a User extension with a license assigned to it.

Can you go to Phone System > Phone numbers > All numbers. Find the number you want and check the columns | Assigned to | Ext. | and most importantly the column | Type |.

If we are using the same language then the number you want as outbound Caller ID currently either has Type: Softphone or Type: Desk phone
If it has a different type. then please share what type it has currently and I think I will be able to give you a step-by-step.