The phone number, that is. The one you dial to see who last phoned. Useful, but it can be annoying. I've just had the following experience (rather reminiscent of Winnie the Pooh).
Caller: it's not Toby. It's Tim.
Me: hallo, Tim.
Caller: who is that?
Me: It's Fran.
Caller: did I ring you?
Me: yes. The phone rang, and I picked it up, and it was you.
Caller: did you ring me?
Me (slightly exasperated): no. You rang me.
Caller: ah. I just dialled 1471
Mystery solved. John had called Tim earlier, and not left a message, so Tim wanted to know who had called. I just wish people wouldn't do this. I feel all confused now (so, I gather, does Tim).