You would need a dropcharge dialer, this way no matter the duration of the call you will be paid a set fee. Short holdtimes are not really profitable on perminute calls these days, because it only adds up to a buck or two w/ the current rates. This is usually the case for shitty dialers w/ poor content or a poor setup.
The conversion ratio is hard to be determined, I usually send 404 traffic to dialers and the return is pretty good vs pay per signup. Clicked/targeted would of course convert a lot better.
I'm currently with
THESE GUYS
-Converts pretty good, I'm currently using the US Drop Charge Dialer they offer ($5 Per Call)
-Allways got my payments via PayPal np
Hope that helps alittle