Arguably you are looking at this from the wrong point of view , you can receive calls from mobiles and (presumably) the majority of regular landlines , but you have a problem with certain calls from some particular places, there are hundreds of suppliers of ‘Telecom’ suppliers , there are dozens, if not more ‘wholesale’ network providers , and especially large scale business, tend to shop around for providers, BT may be a big player , but they don’t have all the market , probably at round 25% , so a 75% chance the people not able to call you don’t even use BT….the originator of a call has the ‘responsibility’ for delivering that call, a network issue could exist either on the originators platform or the interconnection between them and your provider ( BT ) but any call failure fault report should start with the originating network , which is pretty unlikely to BT.
If the Hospital cannot contact you over your landline , they should ( you would hope ) investigate that with their supplier , even if that requires them to contact the receiving network ( BT in your case ) it doesn’t work in the reverse direction , and never has.
Even if you state the problem only ever appeared after you were migrated to Digital Voice , it doesn’t necessarily follow the issue is at BT’s end, they may have implemented routing changes that some providers have responded to ( mobile operators you don’t have an issue with ) perhaps whatever provider or providers you have difficulty with , have not made the appropriate changes to their own networks.
It also seems something of a dereliction of duty , especially in a medical situation, to make presumably an urgent calls , not get through, and not try an alternative number ( possibly a mobile ) or email or something…and if this establishment had numerous issues getting calls to successfully terminate, would be contacting their supplier to see why this is happening
