Probably the hospital are using some low cost routing that takes them outside of the BT network and, like many things outside of the BT network, is a little bit quirky.
Medical institutions often do withold their number. They want to avoid causing worry to people who get home, find a missed call and, when they identify the caller as a doctor or whatever, start to fret and worry about it.
Within the network, in the days I worked on that technology, the 'international' flag was independent from 'calling party number'.
