Question about GSM frequencies

[AlexSmith]

I have a few questions and maybe someone could shed more light on the topic. My wife is from Medellin, Colombia and the smartphone market has not reached the levels like it has in the US, primarily due to the poverty level. At any rate, they do not use CDMA phones at all. I send smart phones down to Colombia, and hit a brick wall.........LITERALLY. It seems that only AT&T phones are able to work in the homes, due to 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 at all inside. Someone said that they scan the frequencies and lock to 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 more about the phones, especially before creating an international trade between here and there.

[Varjak]

No, you can't change the frequencies; but it doesn't have to be an 'ATT' phone to have the frequencies that work best for you. If you look on Wikipedia, you'll be able to find information for individual carriers, phones, and networks.

[Adama D. Brown]

I have a few questions and maybe someone could shed more light on the topic. My wife is from Medellin, Colombia and the smartphone market has not reached the levels like it has in the US, primarily due to the poverty level. At any rate, they do not use CDMA phones at all. I send smart phones down to Colombia, and hit a brick wall.........LITERALLY. It seems that only AT&T phones are able to work in the homes, due to 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.

Welcome to Brighthand, Alex. Short version: It depends on the phone, but most will probably work.

In Colombia, the GSM frequencies are in two bands around 850 and 1900 MHz. This is basically exactly what AT&T uses, which is primarily 700, 850, and 1900. T-Mobile uses 1700, 1900, and 2100. However, most of T-Mobile's phones--especially recent ones--also support 850 so that they can roam on AT&T's network here in the US.

Here comes the tricky part, though. The Galaxy S and the HTC HD2 both theoretically support the 850 MHz band. So if they're not working in situations where AT&T phones are, then something is amiss. Usually all you have to do is make sure the phone is unlocked and supplied with a SIM card from the local provider.

It might be that there's some setting in the software on these phones that is causing them to not roam onto 850. I would go into the settings > Wireless & Networks > Mobile Networks, then make sure that any settings for automatic roaming are checked, and that any "System selection" modes are set to either GSM or 850/1900.