I think that if you have identified a place where you would like to be berthed and are serious about it - don't ring - go in person.
Many years ago I was talking to someone about moving my berth to the SW (I was based in N Wales at the time). He mentioned Plymouth as a good base but ventured that berths were few & far between for my size.
My wife (pregnant) & I just decided to have the next weekend down there & ask around. Most places were full but at the Mayflower the harbourmaster said there was one berth available for my size of boat. I said straight away that I would take it. I honestly believe that if you're on the spot you have more chance (later experiences while cruising further afield have confirmed this IMO). We had four very happy years there.