Voice is not sent as a series of tones, but coded into data bursts. For information about how this is done, see the Digitising page.
A mobile is only actually logged onto one cell at a time, but that cell may have several other mobiles using it at the same time. A basic single-channel cell uses Time Division Multiple Access (TDMA), which allows eight mobiles to take turns to actively use the channel.
There are around 100 to 200 channels using different pairs of uplink (mobile to base) and downlink (base to mobile) frequencies within each of the GSM bands (900MHz, 1800MHz and 1900MHz). To minimise the effects of interference, the mobile and the base frequency-hop during a call.
Far more than eight mobiles can be logged on to a cell, as long as they don't all want to make or receive a call at once.
To explain it, it seems wise to start with what happens during a call. See the Air Interface - During a Call page for details of this before moving on to the Air Interface - In Idle Mode page, which describes some of what is going on when the mobile isn't making or receiving a call.
When the mobile moves (or other factors require it) the call is passed from one cell to another. For details of this, see the Handoffs page.
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