I have the TP2 on TELUS in Canada and it's a world phone, just like the Verizon version, meaning that it can operate in either CDMA or GSM mode. I can't speak for how Verizon operates, but on order to use a non-TELUS SIM in my phone, I first had to SIM unlock it, which you will be able to do
here (thanks Olipro!). This will SIM/CID/security unlock your phone, allowing you to use any SIM, plus you'll be able to flash custom ROMs. It costs money, it's worth it.
You may also need to flash an unlocked radio in order to use US GSM bands, which you can find
here (again, thanks Olipro!).
So now that your phone is all unlocked and ready to rock, you should be able to go pretty much anywhere in the world, pop in a local SIM and start calling. If you want to get really fancy, get yourself a SkypeIn number
here and forward it to your phone number abroad. Then forward your local Verizon number to your SkypeIn number and people back in the US will be able to call you without paying international long distance.