Originally Posted by TungstenX
Thank you for the reply. I understand it a bit better now. I think my service provider set up the system to catch this, and charge for it :(
Very likely. Some providers aren't but you won't get them to tell you that officially because they may decide to do so in the future.
btw, a cellphone (on GSM) digitize the voice channel, thus your voice get digitized, compressed, broken into packets and streamed. The network it self does a how bunch of stuff with these packets. Then the phone on the other side un-digitize the sorted packets, decompress them and play it to the other person (it is a whole bunch of stuff that happens very fast).
Yes, they're called CODECs and are designed to do speech. A single call could be coded and decoded more than once depending on the hardware on each end. They don't usually perform very well on the kind of signals a modem would use.
Also, did you know that the cellphone operators injects what is call "comfort noise" in this communication channel. Because of analogue landlines, we got used to the "white noise" on the line. Thus if there is dead silence, we tend to think that the call was dropped...
Yes, they also introduce local echo so that we don't yell into the phone, most of this is done as close to the endpoint as possible so they don't have to ship white noise and echos around the world.