First create a fop2 group with your call center agents only, name that group "callcenter"

Then create a fop2 permission: custom permissions should not have the same name as stock permissions, so you can create a permission named "supervisorspy"

Add "spy" and "whisper" permissions in the permission box.

Click on the callcenter group in the groups box.

You will have now a "supervisorspy" permission that you can assign to users, so , go to fop2 users, edit your supervisor icon, remove the spy/whisper permission if they are there, and add the supervisorspy permission instead.

Apply changes, then relogin to fop2 with your supervisor extension, you will be able to spy/whisper only to the callcenter group.