Rothrock is likely close to correct. Many offices of all types have one main phone number, but many lines (extensions). When you call in, they want the call to be on the main line. If a doctor in the office, or a nurse, for example, calls you, they don't want you calling that person back
directly, they want you calling on the main number. So, other numbers are blocked.
It really does depend on the phone system they have set up. For example, in my building there are a over a thousand phones, each with a unique number ending in -4xxx or -5xxx, and each able to receive direct calls (e.g. 617-555-4675). But, all outgoing calls show they are coming from the same number - 617-555-4000 - no matter which line they come from. A different approach to the same situation, i.e. you can only call
me directly if I give you the direct number.