It would be nice to be able to do callback on the number that originated the call and not just on a specific number.
That way one could trigger a callback from a friend's phone when visiting or even, in some countries like UK, a payphone.
Of course PIN code would be needed in that case.
If misuse is a concern it could be done so that one must specify a list of one time PINs that have to be used in order (cannot be reused) and, depending on account type, there could be a limitation to eg. 10 or 20 PINs, which is enough for the casual user making few calls when travelling.
I have written somewhere here on the forum how it can be done through a webpage.
You would fill in the number you want to receive the callback on, you will get the call and are asked for a pin number. Than would would get a dialtone.
The problem with caller id is that it's not conistant. For example I sometimes have the problem that the number shows +316xxx or 316xx or just 06xxx depening on where I am calling from and to. So it's very difficult to base a callback on this.
Also even if there is no misuse I can just call your number and receive a callback and just wait to get disconnected, in the end it will cost you money.
