A lot of doctor's offices or hospitals or even schools use a "dummy" number as their caller ID—sometimes it's the main number, but often a closely associated number. In one case the main number is XXX-8100, and the dummy is XXX-8200.

I always add the dummy number to my contact page for those companies/organizations so that their calls can come through. It's sometimes a problem when you don't know which clinic is calling, but fortunately most leave enough information on a VM if you don't pick up the call.

It irks me, though when someone uses their personal phone to return a call, because that number will go directly to voicemail, and in several cases has resulted in a delayed answer or problem solution. I've warned people about that and urge them to either give me the number they'll be calling from or use their office phones.