Keycloak requirements:
- Create client 'phone-box'
- Set 'Access Type' to 'bearer-only'
- Add role 'reader'
- Add 'phone-box' to client that connects to phone-box
- Create mapper 'audience'
- Set 'Mapper Type' to 'Audience'
- Set 'Included Client Audience' to 'phone-box'
- Set 'Add to access token' to 'ON'
- Add builtin mapper 'phone number'
- Set Scopes->'Full Scope Allowed' to 'OFF'
- Add 'reader' client role under Scope->Client Roles->'phone box'->Available Roles->'reader'->Add selected