These areas might need a contract made between other services to cover while their truck is on a transport. FDs do this. Even SFFD (San Francisco) will use AMR as a backup when their trucks are busy.
We do have mutual aid arrangements with surrounding towns and a private company, if necessary. Unlike some volunteer services mentioned on these boards, we don't have any problem getting a rig out, but it has happened that we've had three or four rigs on the road already and there are physically no more ambulances to send. Then you have to call for extra help.