I've tried many apps for VoIP on Android, but it turns out, there is a pretty hidden feature in the "Phone app" whic allows you to use VoIP in Android without any additional resource killer.
You'll need a SIP account to use this. SIP1 is protocol invented especially for voice over IP and you can get an account from many providers. I'd recommend Linphone2 for start since it's free. Once you have a sip:email@example.com you can call other sip:firstname.lastname@example.org accounts.
Unfortunately not every mobile provider allows SIP protocol on their network, so this method is mostly to be used over a Wireless Network.
Adding the account
Open the Phone app ( with the phone icon )
Go to Settings ( the three dots at the bottom )
Scroll to the bottom of settings
...and add an account
Tick the "Recieve incoming calls" if you want to be able to receive calls, not just initiate them.
Once you're done, you can add SIP numbers to your contacts by selecting the
It will show up like this:
And you'll be able to do free VoIP calls, without any memory hog, resource-eating, privacy intruding app.