The only VOIP app that works with Android right now is Skype, and AFAIK, it's only available on Verizon's devices.
Sprint doesn't have a network in Europe. Sprint phones use CDMA network technology, which is not used in most of the rest of the world. You may be able to roam on European networks with your phone, but it's going to cost you big time. In fact, you'd be advised to not even turn the phone on after you leave the US, because you'll get hit with incredible roaming fees.
If you had a GSM phone, you'd be able to use a local SIM card for phone and text services, and whatever available wi-fi there is for data. But you don't have that option. This is why I try to talk people out of using CDMA services...we're the only country that uses them, practically.
If you have to make calls while you're there, buy a cheap GSM cell phone on eBay (or ask someone with an account on T-Mobile or AT&T is they have an old phone they can loan you). You'd have to get the phone unlocked, but that's pretty easy, and you can then get a local SIM and call and text where you need.
Just remember, overseas and international cell calls are really, really expensive. Avoid doing it unless you absolutely have to.
