Cheapest way to make phone calls from abroad?

I have Google Voice, Skype installed. I was trying to get both installed on my phone. Skype I guess can’t be installed without Android (which my phone SHOULD have but does not, thanks to TMobile’s horrid customer service). GV can’t be used directly outside of the US is my understanding.

From my laptop, I will be able to use Skype but I was hoping to set up some kind of forwarding mechanism for my cell phone to by pass the TMobile roaming charges since the Skype and GV rates are orders of magnitude better.

I’ve read something about SIP?

It seems like with these things there is some way to do it, but I’m confused.

Also can anyone point me to a reference page or something that explains in detail how SIM cards work? Lots of people buy country-specific ones and I’m wondering if using a diff SIM card means using a diff network?