It's possible to record a custom voice prompt and have it play back instead of the traditional voice engine. Set the following:
*Keep in mind that this will override the main menu as well, so you should record the relevant prompts (i.e. press 1 to find someone to talk too... press 2 to find a meeting)
static $en_US_greeting = "https://example.com/your-recorded-greeting.mp3"
You can also set a custom greeting for voicemail.
static $en_US_voicemail_greeting = "https://example.com/your-recorded-greeting.mp3"
These settings are overridable from within each service body call handling.