Receiving calls over Wi-Fi first
You might want to take a look at Asterisk, an open source PBX.  You can create a dialplan that will ring your Skype or sip address first, and then switch to your prepaid phone number. With version 1.8, Asterisk supports Google Voice in addition to Skype. Dialplans are completely configurable, however this does entail setting up a server to run Asterisk. The good news is that the minimum hardware requirements are very low; you can even run it on a consumer router running custom firmware such as DD-WRT, OpenWRT, and Tomato.

A couple places to get you started:


Asterisk: The Definitive Guide - a free book Oreilly book, written for an older version, but still a great primer
Asterisk 1.8 documentation