Since you can't cookie a conversion.. how would I retarget to users who've visited but not converted?
The only thing I can think of is using onClick to fire the conversion pixel dynamically... but that's only if they land on your page. (if they use the click to call, can't even cookie them)
You can't really do it with call-only traffic.
However, you can add the remarketing tag on your landing page for the uses that do visit your page, to create a small audience.