An online system authenticates a user through a voiceprint biometric verification process. When a user needs to be authenticated, the online system generates and provides a random phrase to the user. The online system receives an audio recording of the randomly generated phrase and retrieves a previously trained voiceprint model for the user. The online system analyzes the audio recording by applying the voiceprint model to determine whether the audio recording satisfies a first criteria of whether the voice in the audio recording belongs the user and a second criteria of whether the audio recording includes a vocalization of the randomly generated phrase. If the audio recording satisfies both criteria, the online system authenticates the user. Therefore, the user can be provided access to a new communication session in response to being authenticated.