Since posting my message, I discovered that I had another answerphone - a more up-to-date Panasonic digital type. This has the feature that I needed - it is called 'Toll Saver'
I think that I am going to have to replace these units at some time in the near future. Has anyone any recommendations (that still have the 'Toll Saver' feature?
I am going to tick 'Resolved' but please feel free to offer soem recommenadtions.