Where I live is "3" a UMTS operator. Remember that GSM and UMTS phones behave differently and could give very different results when compared. Do also bear in mind that the operators may not share the same antenna sites. Operator "A" may give superior results with a small handy inside the vehicle, while operator "B" may not have a favourable site just where the vehicle is positioned during the test. The only comparisation that could really be made is moving the SIM card between different phones assuming same operator all the time. The propagation on these frequencies depends very much on reflections and also absorbtion that for instance is quite high in vehicles equipped with infra red heat protection (Thin metal layer on the wind screen). Therefore if the base station is in the front of the car and a handy is used inside - don't expect to get good signal strength. Portable GPS navgators with internal antennas will not work very well on the dash board either. Therefore my recommendation is to always use an external antenna "shark fin" on hidden in the pattern of the rear window.
