The centre section is the receiver, containing several large filters which allow the receiver to reject signals outside the required channel.
Cellphones use a large number of closeley-spaced channels, so extensive filtering is required.
This section also contains a highly accurate crystal oscillator, which is temperature compensated to ensure the transmitter and receiver are tuned exactly to the correct channel, to within a few parts per million. The red device top-right is the SIM, a smartcard chip containing the user identity. Extensive use is made of custom chips, due to the huge production volumes, and some of the components are almost invisibly tiny - most of the resistors measure 0.5 x 1mm. Metal can bottom left houses more of the RF circuitry. 8KByte memory device left-centre holds user's stored numbers and option settings.