That's what I like about TMobile. We have a lot of lines on our account and they provided us with three (yes three!) femto cells which use existing internet connections to produce a tiny (hence femto) cell site indoors. Coverage is decent and you don't need ANY signal at all period.
The other option (and it's not free by any means) is a classic signal booster. You still need a signal at your location at least outdoors. (say one or two bars). An antenna is mounted on your roof (or in an attic) and the indoor antenna or antennas get mounted indoors as far as possible from the outdoor antenna. What this does is takes the weak signal outside and significantly boosts it indoors. Wilson makes good ones. If you're handy or find an installer and don't mind spending a few hundred bucks for the hardware, it's a good alternative to the hassle of switching carriers. Particularly if the switch is more expensive with recurring charges. When we had a signal issue and femto cells weren't practical (our wired connection wasn't the greatest) it worked great. Just be sure if the outdoor antenna is on your roof that it's properly grounded per code which an installer should take care of.
In either case, having 4-5 bars everywhere in your home, even in the basement is nice. No missed calls again!