So all he has to do is phone up the old number, find it’s wrong, re-connect to the operator and ask for Harriet by name. Simple! Harriet is also apparently able to ring Peter’s flat without needing to look up the number (on the assumption she hasn’t got it memorised!), so “Wimsey, 110a Piccadilly” doesn’t seem more embarrassing than “Mayfair 1281” or whatever it would be.

The point about it being harder to make malicious phone calls via an operator isn’t something that had occurred to me, but makes sense.