Today, many businesses are looking to create custom VoIP calling apps to enhance communication and collaboration among employees in the digital world. With employees becoming more mobile, there is a growing need for unified communication tools such as voice, video, and chat while on the go. To meet these needs, companies are seeking white-labeled calling apps that are tailored to their brand and workflows.

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SIP enables you to incorporate Voice over Internet Protocol (VoIP), allowing your organization to continue using its VoIP supplier. SIP trunking allows you to give multiple simultaneous communication channels to your customer or company.

https://www.apphitect.ae/blog/....best-sip-calling-api

The Session Initiation Protocol, sometimes known as SIP, is a type of signaling system used to regulate communication sessions. These conversations can be conducted using VoIP (Voice over Internet Protocol) or other platforms, which are often online or cloud-based. SIP is simply one technique of installing VoIP; its main advantage is that it establishes a direct connection between private or local telephone systems and the public telephone network. Individuals and corporations can now connect without relying on a traditional telephone line.

https://www.apptha.com/blog/be....st-sip-calling-softw

To create a VoIP calling application, you must: choose a development platform (iOS, Android, or cross-platform), choose a VoIP signaling protocol (e.g., SIP), include an appropriate audio codec (e.g., Opus), create the user interface, include basic functionality such as setup of call, audio streaming, and call management, and include appropriate network handling to ensure the call quality in different network conditions; this typically means using APIs of the target platform and a backend server for connection and user data management.

For more details for step by step process: https://dev.to/alexsam986/how-....to-build-a-voip-call