At our house the tower with the best signal is a TMobile tower on the Extended Network, but my wife's phone struggles to pick up a weak Cing owned tower and as a result frequently drops calls, then momentarily looses service before picking up the TMobile tower. When on the Cing tower at home the phone gets 0-1 bars but full signal with the TMobile tower... Would a 64K sim allow her to just pick up the better signal all the time?

Or is there some other problem here? Why cant the call just jump from the cing to the TMob tower if the signal is weak rather than dropping/loosing service momentarily?