The voicemail service can use text-to-speech (TTS) for several functions:
Automatically provide an entry prompt for actions using the action’s description text.
Add spoken prompt’s during a call flow using Speak Text actions.
Perform email reading.
The speech engines used for TTS are provided as follows:
Locally installed – Using services installed on the same server as the voicemail service.
Google speech – System using subscription mode can be configured to use Google speech services to provide TTS. This overrides any local TSS service.
The Google speech service also provides the voicemail service with support for a number of automatic speech recognition features.