I've been fiddling with SIP. It turns out there are a number of companies that will allow you to create an internet only account for free, with I presume the notion that you will eventually buy their service. You can just do an internet search on "free sip account" and they will turn up.
Now you can proxy your sip, though I'm not sure how much that does to protect your identity since you still have a SIP address or phone number.
The Guardian has some info on GSM security:
It seems to me the cellular provider can eventually start to add jitter to the data service to stop people from using sip over their phones.
Here are some test numbers:
If you want to experiment with SIP, I'd suggest doing it from a PC first since those apps are more mature. Twinkle is a good app on Linux. It even handles multiple calls.
The sip cell phone apps give you the option of using wifi or the cellular network.