Before deleting the group, make sure that all attachments are disconnected from it, namely:
from the incoming or outgoing direction of the number.
from the IVR (interactive voice response menu).
Disconnect from all attachments, and then you will be able to delete the group
How do I delete a group?
Go to the Group.
Click the button "..."
Click Delete.
In the modal window that appears, confirm the action with the Delete button.
