K8y
For VoIP, there are two that get discussed pretty often (particularly for US based customers):
cheogram/jmp.chat (does not require google play services)
mysudo (requires google play services to be installed if you want incoming call/sms notifications; also requires you to sign up/pay subscription fee from another standard android/ios device)
If you search for either of these on the forum you'll find quite a few threads on them!