BVR allows caching of TTS utterances to reduce load and network traffic to the TTS server. This is supported when using IBM Watson TTS, Amazon Polly TTS, Google TTS and Eleven Labs TTS call features:
Caching of these resources is not controlled through Cache-Control headers. Instead, their lifetime in the cache is determined entirely by the resource_max_age setting: TTS Cache Configuration Options
By default, TTS caching is disabled. To enable TTS, set the size_on_filesystem parameter in TTS Cache Configuration Options to be a value other than 0.
On Cache Management Cache Management Using BAM
On TTS Cache configuration TTS Cache Configuration Options