Correct I am with T-Mobile and may switch to AT&T.
A few months ago I switched from AT&T to T-Mobile to save some money. Right away I saw my phone was not working well inside buildings. I did some research and many people online and on forums said it's probably because T-Mobile and AT&T sell the Intel modem iPhones. I was thinking my issues were either that or T-Mobile just really sucks indoors. So for the past 7 months I can't use my phone inside Target, restaurants, hotels, etc (unless of course there is wifi).
I told myself my next phone won't be one with an Intel modem inside just in case. Just want to eliminate any issues. I never had those issues with AT&T so my thinking was I'll switch back, and I may even try out VZ if not satisfied with AT&T (dead spot with AT&T my wife goes to often and I can't get a hold of her). The Qualcomm one would allow me to use AT&T and then VZ, right?
it's not the modem, but T-Mobile doesn't have a lot of low frequencies which penetrate walls. Verizon has the most.