I have done this and it works, at least on android browser and chrome for android. Haven not tested yet on iPhone/iPad.
Exactly as Firewires said, just use the browser object, open URL -> "tel:12345678" <- use as link -> triggered by touching on certain object, etc
Now when I tap on the number it will bring up my dialing options, add to contacts, etc.
Make sure you put "tel:" before the number, that's important!