The dialer is basically a giant asynchronous state machine with a lot of moving parts under the hood, the API you consume does a lot of orchestrating in backend.
The dialers API has 2 parts, the RESTful section, which you use to issue commands to the dialer and a notification system.  

RESTful API

Use the RESTful API to issue commands to the dialer, do not use this to check state, its an anti-pattern and will result in many hard to debug bugs in your application.

Notifications

We use redis's PubSub and Lists for notifications, because it's the simplest queueing mechanism for integrators to integrate with, and is very low latency. 

Basic Flow

The basic flow is that you will issue a command to the API and wait for a notification to confirm that that state change has happened.

So if you want to add an agent to a running campaign and make him available for calls, wait until the dialer sends you the notification to say that the state change has succeeded, instead of assuming it has after doing the RESTful API call. 

Conclusion

In closing react to the notifications and trust that the dialer will manage the state properly