From Belledonne communications:
Linphone is an open source app offering free audio/video calls and text messaging. With Linphone, you can be reachable at any time, even if the app is closed, with a WiFi or 3G/4G internet connection. Linphone key features: High definition audio and video calls. Audio conference calls with various participants. Pictures and file sharing. Linphone accounts reachable at any time, even if the app is closed (thanks to push notifications). Smart contact list to know whom among your friend is reachable thanks to Linphone. Secure communication (encryption options). Compliant with a large number of SIP-compatible VoIP service providers allowing to reach everyone that has a "classic" phone line. Warning: Since IOS10, SIP connections in background are no longer allowed. Therefore, only Linphone accounts can receive calls when the app is closed, thanks to push notifications integrated within our server. It is possible that ending a GSM call to answer a Linphone call, through the double call screen, may lead to a loss of sound during the call. To get the sound back, you just need to pause and resume the call. Linphone is available on the main mobile and desktop operating systems and offers various specific features for advanced users (codecs, transport standard, encryption options, DTMF.). For more information, see Linphone project website: www.linphone.org. Note for developers and professionals: Linphone can be rebranded and customized to meet specific needs. Contact Belledonne Communications www.belledonne-communications.com for any specific development query.