If the inmate sent a message to your phone number, their message will come through like a regular text message with a link for you to open and view the message. In that same area it will provide you with the option to respond to the message.
It will be the same if they sent the message to your email, except it will be sent to your email inbox.
*Please note, some facilities do require you to have funds on a Personal Tech Account before you will be able to respond to an incoming message.