Using twilio calling a phone number is relatively trivial, one can use a number element in a twiml response. But using the client tag is a little more complex, because twilio has to know how to map that client id to a particular callee's device or browser. In my current web project I'm trying to figure out how to call a particular mobile app user without using a phone number. The mobile apps have no problem calling each other, but I'm trying to figure out how to call the mobile app from the website.
The mobile app is probably based on a twilio android kotlin example with a solution based on a twilio nodejs server example that provides the twiml to the twilio cloud. However, in my web app when I use the same client ids that the mobile app uses, I'm not getting a response in the mobile app.
So that leads me to wonder:
How does one register a client id to be called with the client tag?
Also additionally I was wondering is there a way to get a list of all the client ids registered with twilio?
The clients get their identity when they obtain their access token.
As you stated you use the <Dial> verb with the <Client> noun to call the client based on the identity they obtained via their access token.
There it not a way to list all registered Twilio clients in an account.
You can use the <Dial> action URL to determine call disposition via the DialCallStatus return value.