AT&T LTE 700mhz shows coverage in Charleston, SC. So maybe an AT&T LTE phone on Straight Talk would work. I thought LTE coverage was less available then 3G though.
3G "reaches" further and covers more than LTE (with the same frequency/bandwidth), though LTE would have more throughput per unit of frequency....
as someone said, AT&T doesn't have much VoLTE support yet(T-mobile has 15 markets so far and 4 phones, AT&T has a couple of markets and one? phone )
you could check if Sprint has coverage and roaming on your area... Sprint phones sometimes roam on Verizon's network (dunno what are the circumstances where you are allowed to roam / roam limits)... but give your light data usage, Verizon's prepaid plan works out for you and gives you the necessary coverage/building penetration you want
Maybe I am not using that map correctly for 700mhz LTE on AT&T. When I select AT&T 700mhz 716-722 the entire US is covered with no gaps?
these frequency "blocks" are auctioned off differently... they may be by state/local/"regional"/national blocks, and everyone can bid on several of these blocks (or other rules)... if you look at the different frequency bands, the map can be sliced up differently
it's possible for AT&T to "win" the entire US region for 700mhz band D block (in reality, AT&T bought this from Qualcomm, and qualcomm wonit in 2008, see link below)
700mhz band D is unpaired, meaning it's only one-way broadcasting... that's why it's less desired and AT&T got it because it's cheap and AT&T has some specific plan to use it
most bands are paired - eg: AWS has 1700 used for uplink, 2100 for download
http://www.phonescoop.com/articles/article.php?a=187&p=232
http://www.phonescoop.com/articles/article.php?a=187&p=233