Landline (VOIP) Caller ID

I need to get the caller ID phone number from a VOIP phone line so I can input it into an Arduino.  So I was thinking why not just buy one of those  $20-30 caller ID boxes that display the caller ID number on the box's LED and instead just input it into my project. I just don't know how to do it.  I'm sure the schematic for the box in question would be needed.  But that is as far as I go.  If someone could help me I will get a box with the schematic that belongs to it.  The arduino project is already programmed Thanks in advance  -Ken