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The very easiest is to use the built in Flite TTS engine. It can speak variables.
https://wiki.freepbx.org/display/FPG/Text+to+Speech+User+Guide
It sounds… not awesome… But you can add voices… -
One the coding side Asterisk comes with SayDigits command in the voice of Allison (standard messaging voice).
That’s great for speaking numbers and letters. -
The one I like a lot it the Polly TTS API integration.
https://aws.amazon.com/polly/
TTS Engine Custom - Amazon Polly - 24 languages
@jersonjunior has been a hero on getting this to work for the masses. I hope one day it can be incorporated as standard.