Main Company Number requried for Authentication

If you use the main company number to login with the super admin (normally the 101 extension), then you don't need the extension number. If you use the main company number to log in another user extension, then you must specify the extension number. Remember the password must be the password of that user extension.

One way to double check if you are login with the correct username and password. If you can login the service.devtest.ringcentral.com, you should be able to authenticate your app with that username and password.

When you wrote this "I am not able to login with the username (email address and not my phone number) and password on service.devtest.ringcentral.com but using same credentials I am able to login to https://service.ringcentral.com/." It means that you are talking about your production environment.

In that case, you have to graduate your app to the production then change these app credentials in the sample code.

const string RINGCENTRAL_CLIENTID = "<ENTER PRODUCTION CLIENT ID>";
const string RINGCENTRAL_CLIENTSECRET = "<ENTER PRODUCTION CLIENT SECRET>";
const bool RINGCENTRAL_PRODUCTION = true;