You don't talk to other app users, you create Telegram/Viber bots and connect them to this app.
Your potential clients write to this bots and you can answer them from this app.
- Viber: create a bot and use the authentication token from https://partners.viber.com/account/create-bot-account
- Telegram: https://core.telegram.org/bots#3-how-do-i-create-a-bot