You can update your availability and welfare status via text message, without having to go to the ARM web service. This document gives instructions on the commands available, and how to send them.
You can also find this information within the 'Help' section once you have logged in to ARM itself.
Note that all commands must come from the number you have registered with the system so that it knows whose records to update. If you need to change your number then you can do this yourself via the ARM web service.