Telephone coincidence

When I was a student in Brighton on 1971, I used to phone home from a particular red phone box at the same time every Wednesday evening. I dialled (this was before push buttons appeared) the usual number 01626 3210, but didn't reach home. Instead I was very surprised to recognise my grandmother's voice; she lived a few hundred yards away from my parents. Discussing it later, we realised for the first time that my grandmother's telephone number was one digit below, at 2109. What I still cannot understand is why the phone system connected the first 5 digits correctly, but the last 4 digits were all one down. I know I didn't misdial - the pattern would have been very different if I had. The coincidences are that my grandmother's number was closely related though without matching exactly, that the incorrect connection happened, and that the phone line was clear enough for me to recognise her voice when I was not expecting to hear it. I have not yet heard that this kind of coincidence has ever happened to anyone else.