Encryption converts the contents of a communication with a mathematical algorithm so that only a recipient holding the correct key can recover the original. In telephony it is what keeps voice calls from being overheard, protects SMS messages, and shields data while it crosses the network.
Mobile voice has been through several generations of it. On 2G (GSM) the cipher was weak enough that off-the-shelf receivers and publicly documented attacks could pull calls out of the air. 3G raised the strength, and 4G VoLTE made strong encryption the norm. The 5G specifications go further by concealing the identifier that names the subscriber, which makes it harder for a device posing as a base station to track a handset. In every generation, though, the audio is decrypted inside the carrier's own equipment, so the carrier and any legally authorized body can reach the contents of a call.
End-to-end encryption (E2EE) keeps the encryption closed between the sender's device and the recipient's, so not even the carrier or the service operator can read what passes through. Messaging apps such as LINE, with its "Letter Sealing" feature, WhatsApp and Signal work this way. An ordinary call over the telephone network is not end-to-end encrypted, so the practical move for a conversation you would rather keep private is to switch to the calling feature in one of those apps.
What people misread is how far encryption reaches. First, if the device itself is compromised, an attacker reads the decrypted screen however strong the cipher is, which makes a locked, updated handset the more useful defense. Second, cloud backups of chat histories, notification previews on the lock screen and screenshots all sit outside the encrypted channel, and those are the routes that leak. Third, encryption hides the contents of a communication and says nothing about who is on the other end. It offers no protection at all against a call that arrives with a forged number (caller ID spoofing).
Threats that cluster around encryption include wiretapping, man-in-the-middle attacks, in which someone slips into the middle of a session, impersonates both sides and relays the traffic while reading it, and interception on public Wi-Fi. On public Wi-Fi the contents are protected as long as the site supports HTTPS, but the shape of the traffic, such as which sites you connected to, is still visible, and there is always the risk of being drawn onto a rogue access point dressed up as the legitimate one. See phone number privacy and the call recording legal guide for more.