Quote Originally Posted by mojojojo View Post
Actually, the remote of our alarm units is using RF signal.

I infer that the RF frequency are too high for us to hear but the mobile phone's speaker & mic somehow captured and transmited it over the airwaves. Therefore, what I believe you should do (if this really works) is to place the mobile phone (the one with the car) as near as possible to the alarm receiver for it to work.
The GSM sampling rate is so configured as to accomodate only frequencies of a human voice, so if the sound energy is too high/low for us to hear, the technology prevents it from being transmitted.