Linphone

Linphone is an open-source Voice over IP (VoIP) softphone company that offers a range of communication services. Their software, Linphone, serves as a SIP client, allowing users to make HD voice and video calls, engage in one-on-one or group instant messaging, and participate in conference calls. Linphone's application is available for iOS and Android devices, making it accessible to a wide user base. The company provides a free and accessible solution for businesses, call centers, and individuals looking for efficient and reliable communication tools. With its open-source nature, Linphone allows developers to modify and customize the app according to their specific needs.

https://www.linphone.org/