Make sure you get the phone unlock code from Tmobile ahead of time and unlock it. If you don't absolutely need a sim at the airport, 7-11 should have local sims.
A local sim is likely your best bet cost-wise. The calling rates, even to the US are usually pretty good, and local calls will be cheap. I think I paid 99 php for a Globe sim 3 years ago. A couple hundred php of credit added on left me with so much at the end of 10 days that I downloaded a game for my phone at the airport on the way out.
There are "international" sim's that work around the world, but those are so expensive I don't see the attraction unless you continually hop around several countries and want one number. Google if you want one of those. I think their rates run 1/3 to 1/2 the cost of just using your US sim. And they usually cost a lot up front.
Other things you should do is google for the APNs of those providers and make sure to have Opera Mini installed so you have a chance of maybe using mobile internet. As mentioned, you won't get BB data, but might be able to use Opera Mini and apps like the Gmail app for some email checking. Of course if you can find a wifi spot, you'll be able to use your US sim in the 8320 and UMA for free.
Here's a website that may have the APNs, although for another country it had an APN that didn't work.
GPRS settings for various networks worldwide