Fpop's service is data-based with their app controlling voip calls and texts, uses sprint's network primarily but they've got a global gsm sim going now (uses 3 network from the UK, latency isn't horrible but still need their app for calls). Free service is 200min/500text (voip-app based), 500mb, (gsm sim is 200min/mb and 500text, or 200MB if data-only since they have both on sale at different times) but they activate a number of their premium features upon purchasing anything, so you need to log into your account and turn it all off immediately or else start paying monthlies. Turning things off is pretty easy but they purposefully made some of the links small and easy to miss. You can also add people as "friends" to increase your monthly free data cap, by up to 500mb. There's a giant thread on slickdeals.net for getting freedompop friends for this, i've capped a number of my accounts from there within a couple hours of posting the account email. Fpop forums:
https://forums.freedompop.com/
Fpop also tends to have decent used phone deals, like for older samsung galaxy (3-6), iphone (5, 5c, 5s), moto e, etc, which are usually posted on groupon, woot, and a couple other places at decent prices (grade b quality, so expect a quite used phone but sometimes it's better). They support a decent amount of android/apple sprint-cdma phones, and their global gsm sim works on anything after it has been activated (I have my data-only sim in a windows phone, no windows fpop app mind you, and a talk/text/data one in an old iphone4). If you have an old iphone4 (not 4s) and it's on the last ios version (7/7.1) you'll need to jailbreak it in order to put in the custom apn settings required if you get a global sim, because apple thought you would never need this. You can get as many of these things as you want, but make separate accounts for each so you can shift data around (you can share data with fpop friends, up to 100mb one time per month).
Ringplus uses non-voip calling/texting but injects audio-ads into your calls in place of the ringing-tones (ads are very short, delays your call by maybe 2-5 seconds before other side rings, can customize the ads to a certain degree (like can be music or news and such)). Also uses sprint's network, and only sprint. Ringplus is a little peculiar in that for the last year they've been having a lot of different promotions on free service but the promos only lasted like a week to only a handful of hours. We're talking pretty massive free service promos in the past, like 1500, 2500, 3k, 5k+ min/text/mb month (free).
https://social.ringplus.net/ for their general forums,
https://social.ringplus.net/categories/community for their discussion forums, there's a lot of things you have to consider when trying to get into this now. Sprint didn't like it when ringplus was registering pre-paid cdma phones from other carriers, like boost/virgin mobile (technically any carrier could do this, but r+ promos were making them sell-out of those cheap prepaids everywhere), so made it policy to have the phone in service for a year before it could be used on another carrier, so those nice $30 lg volt prepaids in stores won't work on ringplus without a year of service on boost (whoever is on the box) now.
In total, I have like 8 free service plans across both fpop and ringplus (impulse buys, I have a weak will toward this sort of stuff
), so if you're willing to look at the fine print and dig through their account settings to turn crap off, you can get decent service. Otherwise, you'll get headaches, stuck with monthlies, and no real phone support. Not that you'd have support outside of those couple forums anyway, and barely.