"Have you been able to call the 111 number before from that address?"
I don't think we've ever had to use it before, so I don't know if it's always been a problem.
"Have you moved house or changed telephone provider since your last call to 111?"
No, we've been Virgin for years, with the same number.
We're in England though, so there is no reason that it shouldn't be available. It's not a problem with the handsets, as we've tried swapping them for different models, and none of them can dial it, so it's most likely something to do with Virgin.