According to Zoom:
By default, Zoom encrypts in-meeting and in-webinar presentation content at the application layer using TLS 1.2 with Advanced Encryption Standard (AES) 256-bit algorithm.
For dial-in participants joining by phone, the audio is encrypted until it leaves Zoom's data centers and is transferred to the participant's phone network.
Encryption can be required for H.323 and SIP devices joining Zoom meetings. This setting is configured at the account level, group, or user level. Once enabled, encryption will need to be enabled on these devices when joining your Zoom meeting or they will receive an error and be unable to join.
Please see their full article for more information.