How to set up a Linksys SPA 2102?

I have a SPA 2102 and am using it in conjunction with Call Shop software. The problem is when the booths are 'off' the ATA unregisters and won't re register without being disconnected from the mains and powered up again. I have been told I need to extend the 'stay registered' setting but don't know exactly where or how to do this. Can you exactly tell me, blow by blow and in simple terms, how to do this. Thanks.