There are many Android apps on Google Play for Messaging. You can use them free to text your friends or buddies. But there are not many apps for calling features. You can't make call using apps like phone, there are few apps which can support call feature but that can't be free. You'll have to pay for apps having call features. As I said there are many apps and few of them have become very popular among all. Here is the list of apps :
Kakao Talk etc...
Some of the above listed apps offer you voice chat that means you can just record your voice and you can send to your friend and your friend can send you their voice back, that's the way it works as voice chat.
I'd not recommend you to use some stupid apps which says it will save your battery. The basic thing is the running process and usage of device. If you do multitasking it will consume more battery. And the best thing you can do is that always terminate/stop the apps which you're not using at that moment. By controlling your process and running apps on your device you can save battery on your own. And this is what the battery saver apps will do with your phone. May be, instead of saving your battery the particular apps may pop many ads on your device. So it's not recommended to use any free battery saver apps.
"People says 143. I say 444 i.e. Love Your Life"