Telephone Call and Electromagnetic Waves
You are correct, a cell phone antenna is bombarded constantly with radio waves from other phones, broadcast radio and TV stations, satellites, wifi routers ... it is a very long list.  Several systems work together to single out the one signal meant for your phone.

The antenna is an efficient receptor only for a narrow range of frequencies.  Next, the receiver circuits use filters to further exclude unwanted signals.  Even if things are narrowed down to only one signal, there could be many phones using that frequency at the same time.  But each phone has a unique digital identifier, part of which is stored on the SIM.  There is a lot of digital signal decoding involved, but each phone is able to separate out only the information that has a matching identifier and is meant for it.  That information is digital data that represents the audio telephone signal.