[user5994461]

Old houses should have 1 extra socket in the master bedroom at the very least, because the master of the house was expected to plug a phone in there, back in the days. (my parents and grandparents all have one).

Incidentally, this is likely to be the furthest room on the furthest floor, so it can be a good place to add a wifi access point for coverage.

[hnlmorg]

Depends on what you class as “old”. Remember that a great many British homes are 50+ years old. You certainly wouldn’t have considered having multiple phones in a house when they were built. So the extra socket was added after it was built.

Adding extension sockets was a very easy job. So easy that many homeowners did it themselves.

So it’s very likely your parents and grandparents bedroom phone wasn’t part of the original wiring.