Every mobile has a Radio (Transmitter and Receiver). For Example. GSM handphone will have a GSM Radio, Base band processor. The baseband processor are designed to handle the GSM band signals (single band to quad band as the case may be). Each band wil have a frequency spectrum to work with. When the person A calls Person B, the person A phone sends the Caller ID, Destination ID and necessary information to the nearby GSM tower. From there it is sent to the base station which identifies the recipient of the call and send signals to the tower at which the recipient is available. From there Person B gets signal. The base station knows the status of each and every switched on mobile equipment and its location (Cell ID) . By this the call is made.
In case of SMS, the message reaches the SMSC (Short message Service centre) and joins the SMS queue. (Coz many would be sending SMS). When the turn comes, the message is tranferred. Because of these reasons only during festive seasons , the SM