If you want to reply an SMS do the following procedure:

Create a text message starting with the TextID of the sender and then the message. For example, if you received a message from TextID 'justme', you can reply by sending the text 'justme Let's chat online'.

You then send the message to 69999.

As soon as your message is delivered you will receive notification of this to your phone.

Please note that before sending SMS chat messages from your phone you need to add your phone number to your profile.