SLA Question

I have tried setting up SLA on my test asterisk system in preparation for our 1.4.x upgrade. I am having problems getting my head around the concept. I understand how it work on Zaptel trunks but not on IP trunks.

Basically our system has a single VOIP Gateway (Vegatream) with 4 BRI ISDN ports for connection to the outside world. A single SIP trunk is created between Asterisk and the Vegatream box.

My goal is when an incoming phone rings it works the same as an old Key System. A light flashes or lights up on the phone and when the call is placed on hold it can be picked up by pressing the “Line X” button on any phone. I know I know it seems to pointless but our users have tried the new setup and they just cant get used to it and complain continually.

I have looked at the sla doumentation inside the Asterisk Tarball as I cant seem to figure out how to do it for my implementation. I guess my problem is understanding how I willl have multiple lines showing in a single SIP trunk…

Can any1 provide some assistance??