Is it necessary to select a carrier when buying an iPhone online from Apple?
The reason for this is that in the US carriers have and use slightly different technologies and different frequencies than their competitors. Though I believe T-Mobile and AT&amp;T use similar technology and frequencies. Not sure abut the other two.

I would suggest checking the website of the carrier you use at home and see what they use so you can select a phone that is the most compatible with their cellular network.