Smart phones have a variety of antennas embedded in them; such as, voice, wi-fi and GPS, but a cell phone is nothing more then a two way radio and as such, it has a particular frequency that is uses. Each cellular carrier uses a different frequency; such as, 1900 MHz frequency that T-Mobile uses.
All cellular antennas ‘listen’ to the multitude of data that is broadcasting at the carrier’s frequency, as cell phones wiz by them. As the cell phone moves further and further away from the tower, the amplitude (also called ‘signal strength’) of the call diminishes. When the tower senses that the signal is getting weaker, it transmits the call to the next tower that will contain a stronger signal.
As cell phones pass from one tower to another, the signal is also passed on
You can see this action on your cell phone as the signal bars increase and decrease. Poor reception (as seen on the phone as less bars showing) would be the result of some signal interference between the cell phone and the tower and/or that the next tower that is going to pick up the signal is far away.
Cellular antennas are full duplex devices.