The system is on a mobile TV production truck, and I often have different groups of users on the truck at a given day. They want the system to resemble a key and lamp system for simplicity, and they only have analog lines. We do use SCN to tie this truck into a larger company system via VPN.
I have the system set up this way:
The phones have one call appearance on the first button, then the rest are line appearances to grab the analog line directly and so they can join an existing call with other users easily.
For each line, set the prefix to "9". Under the Analogue Options tab, set the trunk type to Loop Start so that it does not wait 3-4 seconds for the Caller ID that will never come with a ringdown. Make sure Secondary Dial Tone is unchecked.
I have a system short code 9N; Dial; N; 50:Main
The ARS 50:Main has the Secondary Dial Tone unchecked, and has these short codes:
11; 911; Dial Emergency; 0
911; 911; Dial Emergency; 0
N; N; Dial; 0
One other change I made was to set all but the first 4 lines to have a different outgoing group ID so that a user can just pick up the phone and dial 9 for an outside line using the call appearance. (First 4 lines use group 0, which is what the short codes point to.) The problem I was having was not all 16 lines may have dial tone at a given location, so they system would time out waiting for a line when there was no dial tone on it. Now, the system picks from the first 4 lines, which usually always have dial tone. If they want to use line 5, they can simply select that line appearance.
I welcome comments from the group if there was an easier way to achieve this.