Callju

With CALLJU Softphone, callers can make calls with their standard cell phones via 3G, 4G/LTE, 5G or WLAN. There are no advertisements.
CALLJU works like standard softphones: it registers with the server and then allows the authenticated user to make and receive calls.
Users can fund their account with credit card or vouchers to make calls. You can even transfer your credit to friends and family in any region of the world.

This softphone turns your smartphone into a VoIP phone, so you can call and even receive calls from your friends and family over the Internet at any time and from almost anywhere, super cheap or even free.