Hi
We have a database that does this in a very simple manner - if I can apply it to yours the cost will be much less than stated.
Basically our VOIP system (Draytek) comes with software that FileMaker can integrate with by using the dialer script function.
When you hit the dial button, it routes the call through to your desk phone (in our case this is a cisco).
My company specialises in VOIP also, so if you can let me know what your VOIP service/system is I can find out the quickest route.
Any questions let me know.
Many thanks