A method and subscriber unit provide mission critical talkgroup service while minimizing truncation and delay when at least one member talkgroup radio is on a Broadband LTE (Long Term Evolution) system. A dual-mode subscriber unit preferably obtains LMR coverage map data to predict the entry point and remaining duration for when the subscriber unit will operate in the backup LTE system. The subscriber unit falls back to the LTE system when the subscriber unit is out of the coverage area of the LMR system. Because the subscriber unit set the connected state prior to using the LTE network and continuously optimizes RRC mode parameters, audio truncation and call delay are greatly reduced without draining the battery of the subscriber unit.