You might also try adding a terminating extension group.
The differences are subtle, but important between a terminating extension group, and a coverage answer group.
A coverage answer group can contain 8 members, a terminating extension group can contain only 4.
The terminating extension group is the principal for coverage purposes, so you can control the number of rings at the extensions. Using a coverage answer group on an x port, you use the "subsequent don't answer interval", which is a system wide setting, because the call cannot ring at the principal station.
People in a coverage answer group can remove themselves from the coverage answer group (so coverage answer calls will not ring on their staions) by pressing the "send calls" button on their station. You cannot remove yourself from a terminating extension group.
Hope this helps.
Carpe dialem! (Seize the line!)