So if you own a t-mobile phone it is unlocked already? I was thinking of buying a phone in SP for the 3 months we will be there but if anyone knows if the t-mobile will work and by this I mean I only want it to work on the island by buying minute cards for it.
I don't think you understand what "locked" means. I don't blame you, as it's not explained anywhere and it took me ages to get my own possibly incomplete understanding.
If the phone is "locked", and most new phones sold to work with particular service providers are, then it will work ONLY with a SIM card from that provider. Even while still back home, if you insert a card from another provider it will NOT work.
SO, even if your phone is locked it will still work in another country so long as the original SIM (or another from the same provider) is in it. And provided the phone can physically access the network, which brings us back to operating frequency. Your connection will only be to your SIM's provider, and if that is by then in another country (eg. you have a US-based phone holding a US SIM but you are in Belize) then you WILL pay international rates. As we've seen, these vary drastically from provider to provider.
If your phone is unlocked then you can put another provider's SIM card in and it will work, regardless of where or in what country you are (again subject to the operating frequency limitations). Wherever and in whatever country you are, the charges you pay will be according to the tariff of your SIM's provider. You may well find that it is cheaper to call your home country with a local provider's SIM card (in Belize that would be Digicell) than with your own SIM card, even though that may work as well. Remember that your "phone number" is actually your "SIM's number", so when you change the SIM card you change the number of your phone.
When you are abroad (in a country different from that of your SIM's provider) there may be additional charges raised by the local service provider which will be added to the base charges your provider imposes. Because the only way your phone can access the outside world is via the local service provider (BTL's Digicell in the case of Belize) and they want a cut of the action!
Beware that many services are initiated by your service provider and not by yourself. The base one of knowing where your phone is so the network can contact it is one of these, but there are many others relating to internet and more. The cost of these is always charged to you, but when you're at home these costs may be so low you ignore them. Once you're abroad, however, the cost of some of these services increases dramatically (see examples people have cited above) and you will be charged these as well. The only way to be sure you WON'T be stung for huge amounts is to find out what these services are (not always obvious, but persist) and turn them off WHILE YOU'RE STILL AT HOME. You will find that some services can't be turned off once you're abroad, as I learned once to my great cost.
The last point I can think of for this treatise is that in the US it seems common to pay both to make and to receive a mobile phone call. That also seems to apply to text messages, though to a lesser extent. In many other countries this is NOT so - in the UK for example, and Belize, you pay to make a call but it costs nothing to receive one. When comparing call rates it's useful to remember this.
In the previous paragraph I've fallen into a common trap. When I say "In the UK" or "In Belize" I mean when you are using a domestic provider from one of those countries. If I use my UK SIM card while I'm in Belize the charges I pay will be determined by that UK service provider, not by anyone in Belize. The charges added by the Belize carrier will be added into the UK charges, not charged separately.
This has been an attempt to explain some of the very commonly misunderstood aspects of mobile phones. The few people who do understand all this stuff often assume wrongly that everyone shares their level of understanding - I know that's not so. But if anyone has corrections or useful amplifications to what I've said please feel free to add them.