You might like to have a look into Doppler effects as you say that the vehicle is in motion. GSM (which I assume you are using) has a maximum velocity specification, but I think that it is rather high, at around 300km/h. Another thing to take into consideration is that while the vehicle is in motion, the channel that the signal goes through to reach your mobile changes very rapidly. GSM phones employ equalisation at the terminals to cancel the adverse effects of the channel (multipath, phase distortion, etc.) and it is obviously more difficult to carry out this process when in motion.
You might like to have a look into Doppler effects as you say that the vehicle is in motion. GSM (which I assume you are using) has a maximum velocity specification, but I think that it is rather high, at around 300km/h. Another thing to take into consideration is that while the vehicle is in motion, the channel that the signal goes through to reach your mobile changes very rapidly. GSM phones employ equalisation at the terminals to cancel the adverse effects of the channel (multipath, phase distortion, etc.) and it is obviously more difficult to carry out this process when in motion.
