A client app can be built using one of the Voximplant SDKs or it can be represented by some SIP endpoint (for example, SIP softphone).
Voximplant provides SDKs for embedding real-time voice, video and messaging capabilities into applications built for different platforms. Currently, there are SDKs for Web, iOS, Android, React Native and Unity. The SDKs use WebRTC for media processing.
In that case, the call processing happens in the Voximplant cloud using VoxEngine scenarios but the scenario usually receives/sends a call from/to the client application.
The integration scheme looks as follows: