I can't find any information on BT supporting the SA box (or not)? Mine stopped working before xmas (its was supplied to me by the company I work for) and I've not been able to find out what I need to do to get it working again?
I assume you originally had it setup by BT -- you need a BT Mobile to use it.
The help page is here:
Set up and use BT Signal Assist | BT help (custhelp.com)
If it has stopped altogether then you need to contact BT on 150 and ask for it to be re-activated.