Here are 3 important configuration rules for ISSABEL call center
1>
The agent extension MUST BE FOUR (4) digits.
2 ( A ) AGENT MODE:
The agents in your STATIC AGENTS box in the QUEUE menu in the PBX GUI must contain the AGENT NUMBER you assign.
NEW AGENT:
https://4.bp.blogspot.com/-ee20z0hx_H4/XExYQgK1NJI/AAAAAAAABb4/3w3-8BQW1IIsMLnouzIYiIfuVnmLQVduACLcBGAs/s640/pic19.png
...then in the QUEUE it MUST HAVE an "A" in front of it.
https://3.bp.blogspot.com/-Z-92hs-kZSM/XEyKp6nq2GI/AAAAAAAABdE/BM5gMxQnoiwNIeqyHr3YJd9rQTgNpELKACLcBGAs/s640/pic23.png
2( B ) CALL BACK MODE
The extension that you are assigning in the queue in DYNAMIC MEMBERS -must- contain an "S" in front of the number, assuming its SIP extension.
https://1.bp.blogspot.com/-x2iaU9s00wk/XEvJjlCF2HI/AAAAAAAABXw/WpODOR-7Hjw_Mhh0Ic2KvzH4YOp46U11QCLcBGAs/s1600/pic3.png
3>WAIT one (1) minute after you configure new agents before testing. There is some sort of process in ISSABEL I think that doesn't always let a new agent work right away. Wait 1 minute between changes to ensure your configuration changes were applied by the system.
This might help:
https://randomcontributions.blogspot.com/2019/03/issabel-contact-center-installation.html
or
https://randomcontributions.blogspot.com/2019/01/issabel-contact-center-installation.html