if you are referring to the Trinity Mother Frances Flight For Life helicopter program associated with Champion EMS, then they are dispatched by Champion EMS on the DFW Communications LTR system. Flight For Life 1 is based in Palestine (DFW Communications) and Flight For Life 2 is based in Winnsboro (their main channel is DFW Communications Purley for dispatch)
closest towers to the Lake Tawakoni area is the Wills Point Tower and the Purley Tower. The helicopters bounce around on the towers so it's difficult to hear them on the system, however they use 155.3400 156.7 PL while in the air and talk to ground units and FD's on VFire 23 (154.2950 156.7 PL)