Go to Cloud PBX, Automated Attendant
Click "Convert text to speech", you can listen to the message by clicking the "Play" icon, after you are happy with the message, click "Save".
Punctuation matters in text to speech conversion. Make sure and enter you commas and periods to ensure a smooth reading of the text.
Save your Auto Attendant, and don't forget to click Cloud PBX - Apply Changes - Provision