Sounds like Cricket is the way to go. I switched from AT&T to T-Mobile back in September when I got my iPhone 6+. At first, I was very impressed with Wi-Fi calling, which sounded crystal clear, particularly when talking to my GF who is now also on T-Mobile.
Unfortunately, over the past few months here in Las Vegas I've been getting a lot of complaints from parties on the line who say they can't hear me, due to frequent dropouts. I also have experienced a lot more dropped calls on T-Mobile than I ever did on AT&T. And as everyone has pointed out, reception inside buildings is disastrous, like when I try to access a site inside Smith's (Krogers) or Home Depot.
Has anyone seen an improvement in call quality when using T-Mobile's own router? I currently use a TimeCapsule to connect to the net. 🙂