Receiving message replies from your contacts is free of charge. You only pay for outgoing messages.
To start a conversation click SMS in the navigation then 'Send SMS'.
Select a contact, send date & time, a template if relevant, compose your message then click 'Submit' to send the message.
Message received by the contact at the exact time you specify to be sent.
Message replied to by the contact.
To check your message inbox, hover over SMS in the navigation and click 'INBOX'. Hover over the received message and click 'Conversation'.
View the historical conversation thread to the left of the message dialogue box. You can then reply to the message accordingly.