on21
If you have "plenty of dead spots" complain to O2, repeatedly if necessary. They have a contract to provide national coverage.
With regard to the "10 minute rule", that was designed into the system deliberately, in liaison with ACPO & HO, to deter unnecessary nattering. If you have an operational requirement for a longer call, then O2 can programme the system to allow your particular radio set do do it. You will have to convince your HQ to authorize it though.