We literally use FreePBX for mainly this function. We went with Amazon Polly for TTS. We added the TTS variables as variables in the call file, then use the Polly TTS command to read it in the dialplan to the callee.
It is very cost effective, and more importantly there is a guide to get it all setup. IT works extremely well. My hope is one day FreePBX will have a module to make adding these TTS services easier.