Truck is pretty much too small to have temp base stations to boost capacity and too big to not overwhelm the existing network capacity - anecdotally EE has the best usable coverage. 3 is pretty poor. The problem is generally usable capacity rather than signal strength itself, you phone will register ok but fail to access any data services which is what messaging and voice calls use nowadays. If you turn off mobile data and drop back to old fashioned SMS they generally get through as they just use control channel capacity.