OK. So Tommy Automatic, my driving instructor, tried to phone me several times today to let me know he was on the way. The call would not go through. He showed up and honked, so I came down and watched him dial my house number. He got "invalid number" back at him.
After the lesson I was able to phone him from the house phone, and he tried to dial me again, but he still could not get through. I tried dialing on the daughter's cell and on my emergency cell and they both got through just fine. (The Daughter's cell is on Sky, as is the home phone, but my emergency mobile phone is on O2 and both rang through.)
No idea what to tell Tommy Automatic! I don't know diddlysquat about smartphones (which he was using). There's not something that would maybe stop him from dialing a landline from a cellphone? (Like how I could not dial a national number from my old EE PAYG phone?)