Talkatone is a free app that gives you a real US phone number for calls and texts over wifi/data. We have it on our phones and it comes in useful. Something to bear in mind - my DH uses Fongo (a Cdn version - have to pay something like $3/mo) for all calling and texting. We have no house phone and use it as our main phone as well. We are saving a bundle with having a data-only phone plan. BUT...call quality is iffy and very dependent on the quality of wifi/data signals. We have had many frustrating phone calls with very choppy connections (think of how your skype is sometimes choppy). If wifi/data signal is not strong enough, you CANNOT make/receive calls. But texting always works very smoothly. e.g. my daughter cannot make/receive calls with her school's wifi (she can at home), but can text back and forth easily (too easily since she texts me ALL THE TIME!). Calling customer service numbers that also use VOIP can get interesting. One time we were in an airport dealing with travel problems and were on the phone with reps and our connection was so terrible that it became a frustrating experience.
Currently, the savings are worth the hassle.
My DH's data-only plan has a few advantages - he can use data on the road, use certain apps, browse on various websites, look up info on the go, no limits on calls/texts, etc. Most of the advantages of his old $60/mo plan. He has 20GB of data that he prepaid for a year, so just under 2GB a month, which is a very decent amount of data. Including all associated costs, we are spending about $15/month on this.
My DD just bought a very basic prepaid plan that gives her conventional calls/texting. She primarily uses her free app whenever she is in wifi but as she's 15, asked to have something that gives her the ability to make a quick call/text when she is on the go (out of wifi) without always having to rely on borrowing a friend's phone. She paid for her phone plan herself - a prepaid plan. It is $35 for an entire year and after all fees are deducted from it, she will be able to send about 6 texts a month on average. Very, very little - but we anticipate it will be more than sufficient. It will also be a good substitute if she is at home, our wifi fails (power outage/tech problems) and she has an emergency.
Oh I should add a couple more cautions about switching to these apps. It is often difficult to receive text messages sent from websites. We usually get password-verification texts (you know - the ones they send you with a code), but cannot receive uber texts (this one is rather annoying since you cannot communicate with the driver to tell them where you are standing - but we suck it up and deal) and maybe one or two other things that I forget. Definitely something to consider.