When creating a Call Queue your Agents are unable to Login and Logout of the queue using the default Postfix.
If you are trying to use the default Login and Logout Postfixes (i.e. 6500* to login and 6500** to logout) the call will fail from a Grandstream IP Phone with the default Dial Plan.
Login to your UCM61xx and setup a Call Queue if you haven't already:
Make sure you have configured the Agent Login Settings as per below:
If using Grandstream IP Phones that are provisioned via Zero Config you can add the following to the Global Policy:
After you have enabled the Dial Plan globally ensure you update / re-provision all phones so they receive the updated Dial Plan.
From one of the phones try the following:
Login to Call Queue 6500: dial 6500*
Logout of Call Queue 6500: dial 6500**