Yes, you can change the voices you want to use in a TTS callout.
For details of how to do this refer to the documentation:
https://developers.sinch.com/docs/voice/api-reference/voice-locales/.
This lists all the available voices and how to implement them.
Documentation for Voice Calling API: https://developers.sinch.com/docs/voice/api-reference/
Android : https://developers.sinch.com/docs/in-app-calling/android-cloud/
iOS : https://developers.sinch.com/docs/in-app-calling/ios-cloud/
Javascript: https://developers.sinch.com/docs/in-app-calling/js-cloud/
Latest Voice SDKs: https://developers.sinch.com/docs/in-app-calling/sdk-downloads/
Voice SDK
Reference Apps: JS, Android, iOS https://github.com/sinch/rtc-reference-applications
Calling API
webhook callback application: https://github.com/sinch/voice-api-webhook-nodejs
php client + samples : https://github.com/sinch/voice-api-php
How to: https://community.sinch.com/t5/Virtual-Numbers/Getting-a-number-from-Sinch/ta-p/8081
Login to https://dashboard.sinch.com/numbers/buy-numbers find the number you want.