iphone mystery

As far as I can tell, one number and one number only, when trying to call me on my iphone 4s receives a fast busy signal. When I call myself from a landline it goes through and I receive assorted other calls, so this does not seem to be a systemic problem. What might be causing this?

Obvious thought: Did you block that number on your iPhone?

Good thought. First think I checked, and I did not. The specific phone number is in a large institution. No matter who calls me from this place the number of origin is the same. When I have the operator call, it goes right through. I assume she is calling on the trunk line. When someone from inside the place tries they get the fast busy.