If you're sending a mass text message out to your customers, they can reply and you'll see their responses within your message report, which you can find out more details about here. You can't reply to their replies though, sorry.

These replies will go into the message report for 72 hours after the message has been sent.

Customers that respond to your mass text message will be sent the Reply Response, which you can learn about here.

Customers can not respond to system responses, scheduled messages to new subscribers, or autoresponders.