encryption drain the battery gsm
"I can reassure you, that in the air it is impossible to eavesdrop your conversation"
This is not quite true. There is one company at least I know of, which makes GSM surveillance equipment, and only sell it to law enforcement. I personally attended one demonstration. This equipment act like a clone to your Cell phone once it is programmed with your phone number, something that involve a court order, that is if you are in a civilized regime, unlike those in the middle east!.
The sale of this equipment to various government officials in Lebanon made a national crises 10 years ago. Various party leaders in Lebanon acquired these, and were listening in on their rival party members, and perhaps you can find stories about this on internet.
The use of BCCH and CCCH (Common Control Channels) to transmit voice back to the base station is very unlikely, since the reverse link, data fields in these channels are bursts, and very small, and even if were used for voice, they would have to sample voice in such way that it makes it unusable. Also, do not forget that it would also drain the battery very quick, I doubt this is possible !. Remember, this field were not designated for continuous traffic, but rather for relaying small network related parameters to the base. Also the power allocation for these was not even specified for good RF link if voice used.
Again, like one of you mentioned, if this were to be done, it would involve firmware and even hardware modification (installing switching and enable lines) to the individual phone, which is a whole different story.
I worked few years at Westinghouse Audio Intelligence Devices in USA, a company no longer exist, but at the time, we made some phone bugs, but that involved installing a small 100 milli-watt additional transmitter inside the phone, which was turned on, the minute you receiver a call. This transmitter would transmit short range to another nearby receiver whose squelch circuit would turn on a tape recorder. This phone was used to combat drug trafficking, and it worked very well and made the company a lot of $.
These products by the way still cell, and it is used very often all over the world by undercover agents.