You can do this by selecting the "I am available" option from here.
Once you have made yourself "available", if you do not make yourself unavailable, you will be automatically be made unavailable after two hours. If you have verified your mobile with us you will be sent an SMS giving you the option to make yourself available again.
You will also be made unavailable, (without SMS notification), if you do not respond to a new Phone Chat booking within 15 minutes or if you opt to reject a DirectChat call.