Query about GSM frequencies

I've a few questions and perhaps anyone could shed a lot more light around the subject. My wife is from Medellin, Colombia as well as the Smart Phones industry hasn't reached the levels like it has within the US, mostly as a consequence of the poverty level. At any rate, they do not use CDMA phones in any respect. I send smart phones down to Colombia, and hit a brick wall.........Virtually. It looks that only AT&T phones are able to work while in the homes, as a result of the homes all being made of bricks...and very thick bricks. What I was told was that T-Mobile phones and AT&T all use 4 different band frequencies, but the T-Mobile is on a high frequency, whereas AT&T uses a low frequeny which is able to pass through walls easier. Now, my quesion is this........Even though its jailbroken, a T-Mobile phone still uses a high frequency; Is there any way to change the frequency of a T-Mobile phone to that of an AT&T phone.
I sent a Galaxy S1 and a HTC HD2 down a couple of years ago, and they could not use them whatsoever inside. A person said that they scan the frequencies and lock on the greatest frequency, but AT&T phones use the lower frequencies. I would like to begin sending high end smart phones to Colombia to create a business, but a person can generally buy a T-Mobile phone used much cheaper than an AT&T phone. I just bought a Galaxy S2 upgraded from Gingerbread to Icecream with a Extended life battery for $125. That same phone from AT&T would cost $300. This is my first post, and I plan to learn much a lot more about the phones, especially before creating an international trade between here and there.