Tracking referrals from the phone number the customer called from based on Referring url

[TravisNPS]

I am looking for a way to track referrals for phone sales, with my phone system I can basically have unlimited phone numbers. So what I would like to do is use some sort of script or image overlay that changes dynamically by refer.

I was thinking I would put a tag in referring PPC URL, so if they were coming from nextag the url would be something like
http://anygenerator.com/1-generac-guardian-20kw-aluminum-w-200-amp-se-automatic-transfer-switch.html?REF=NEXTAG

then when they get to the site if the url contained REF=NEXTAG it would display a specific phone number and they I could report on the phone numbers through my phone system. Any ideas on how to do this?