There are several free and open source APIs available for adding voice call functionality to your application. Some options include:
FreeSWITCH.
It is an open source telephony platform that can be used to build a variety of voice and messaging applications. It supports a wide range of communication protocols (e.g. SIP, H.323, WebRTC) and can be used to build applications that can make and receive phone calls, as well as send and receive text messages.
Kamailio.
Best open source SIP server that can be used to build voice and messaging applications. It supports a wide range of SIP-based communication features and can be used to build applications that can make and receive phone calls, as well as send and receive text messages.
Asterisk.
This is an open source private branch exchange (PBX) system that can be used to build a variety of voice and messaging applications. It supports a wide range of communication protocols (e.g. SIP, H.323, WebRTC) and can be used to build applications that can make and receive phone calls, as well as send and receive text messages.
These platforms can be used to build your own custom voice call solution, but they may require more setup and development effort compared to using a pre-built API. You will also need to host and maintain the infrastructure yourself
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