M1 Messenger is a free and universal solution because it works as a SIP client in push mode: so it's able to receive incoming calls without you having to worry about draining the battery. The main problems with SIP telephony on a smartphone. Your calls are not protected from wiretapping. The service provider can block your VOIP traffic.
The download of this application is safe and secure. Mobile One Ltd is the official developer of the app which can be found in the Social tools category. SIP Messenger has been downloaded and installed by over 10K users and the latest release is dated Jan 16, 2019.