When using the Sinch Voice platform in many use cases a callback server will be required to control the flow of a call.
Why do I need a callback server?
This is for you to be able to have control over certain elements, such as: authorising calls, routing calls, setting a cli (calling line identifier) to the receiving user. It is mandatory to have a callback server for calling a PSTN destination
What do I need to implement a callback server?
You will need an application server that can parse incoming HTTP POST requests and return HTTP POST responses with SVAML commands (json) in the HTTP body. Some example callback severs can be found here: Sinch Github voice samples or for simple testing you can use a service such as https://webhook.site
Where do I configure my callback server URL?
You configure your application server URL in the Sinch Dashboard Voice & Video -> Apps -> In-app Voice & Video SDKs -> Callback URL.