I remembered to saw a long time ago a piece of software that can recognized the phone number. But I don't remember the name anymore (I used to be in ISP business more then 7 years ago). Do some Google research, you never know what on can find ;-)

BTW the callback function has some limitation. I saw lot of case where people use call forwarding (on cellphone most of the time) to bypass the callback function, or more exactly route the call where they want. This can also be use by a malicious user. This is a very complicated attack, but I can happen if the value of the data is high.