Just about any company can get into the mobile operator game these days — they just need to tap into a large carrier’s network to do so.

Today Enflick, the parent company of free messaging and Wi-Fi calling app TextNow, announced a low-cost mobile phone service that starts at just $19 a month. The catch? You’re stuck with aging Android hardware and Sprint’s WiMax 4G network (not its newer LTE network).

It’s not the evolution you’d expect for a messaging app, but it makes sense given TextNow’s recent Mi-Fi offering. That $40 device allows TextNow users to access Sprint’s 3G network with data plans starting at just $15 a month. Now Enflick has a mobile offering for just about anyone looking for cheap service.

TextNow’s $19 phone plan gives you 500 megabytes of data, unlimited texting, unlimited incoming calls, and 750 minutes of outgoing call time. FreedomPop recently announced a similar service on top of Sprint’s network, except it will offer 500MB of data for free. And don’t be surprised if plenty more mobile operators appear over the next year — for carriers, there’s little risk to let one of these companies experiment on their networks.

Your phone choices for TextNow’s service are pretty slim at this point: $90 gets you a Nexus S, while you can get a Galaxy S II for $120. Both phones run Android 4.0 Ice Cream Sandwich. Since the phones are several years old, they won’t be able to run the latest Android games very well and complex apps may bring them to a crawl. But if you’re looking for the cheapest possible smartphone service today, TextNow’s service may be worth looking into.

TextNow also is also working on launching a LTE device, Samsung’s Galaxy S3, later this year.