How do I do this..?!

I want to verify that a user really is the owner of a specific phone number.
This is how it will go:
User dials from the phonenumber they want to verify to us (one number same for all accounts) then enter a unique code displayed at their account.
Then I want the server to send a verification that the phone number/account is validated…

This is the theoretical part, but how do I do this practically?
(We don’t have a PBX and we don’t have a clue how to do this)

Please help me!! :smiley: