Is the signals of a mobile phone-call encrypted between the phone and the mobile-tower or only between the towers? And is it possible for a non-authority individual to eavesdrop on your conversation if he picks up the signal between the phone and the tower?
All normal (GSM,CDMA,UMTS,LTE) mobile network communications are encrypted, except when it is not. For example, very old 2G GSM networks may only offer
A5/1 "encryption", which is in effect none or broken.
Many other protocols are also already broken. See: Cellular encryption algorithms currently in use globally.
Some attacks are done by downgrading the communication from a safe/encrypted protocol to a broken one.