I have an unlocked iPhone 6. I was on Cricket for a while. It's an AT&T MVNO that is directly owned by AT&T. It worked great. It's $35/month for unlimited talk and text and something like 2.5 GB of high speed internet (then throttled unlimited) if you link a credit card for payment. After getting more than 3 users on a plan, the price drops by $5 a month per phone -- you can find a subreddit where people get together to make groups, which can get your bill down to $20/phone.
I've mentioned this on other threads, but around January I moved by iPhone over to Ringplus, which uses Sprint. Each month I get 4000 minutes, 4000 texts and 2.5 GB of LTE data for free. The only catch is you have to listen to ads when you make a call, instead of a ringtone. Oh, and visual voicemail is broken, boohoo. It was something like $35 up front, and that particular allotment was a promotion (they change all the time).