There are tons of Twilio apps that only send transactional messages one way. Delivery services telling users their order is on its way, or booking services sending users a reminder. Normally, if a user responds to those automated messages, their response doesn't go anywhere.
SMS Inbox provides Twilio app developers with an easy alternative: it's an embeddable white-label messaging interface that connects to your Twilio account, giving your users the ability to send and receive messages manually using the same phone numbers as your software's automated messages. It literally takes less than a minute to set up, and you can try it for 15 days free of charge.
After the trial, the price is $75/month for a single connected Twilio account, and $375/month for multiple connected Twilio accounts.