Depends on what you want to use it for?
As publicity or an effect in your act. Also does it have to involve a telephone, be achieved through a telephone conversation or have a theme that ties in with telephones?
Any routine using Psychological Forces can work as will a good number of self-working effects. On top of this there is the Lewis Jone's book "Person to Person" and one I'd forgotten about early entitled "Your Number Please" by Richard Busch
It's for radio. I'm doing an appearance soon on radio and along with performing for them and talking about what I do I want to do some over the phone effects for callers as well.
Like I said I know a few good ones, I'm just trying to see what there is I can find that fits my style that I don't have in my repitoire.
@Craig I posted this on a couple different forums to open up as many possible answers as possible. I'm going to check out every answer I get and decide what to apply to myself. I still appreciate posting it in case I missed it on E though lol