I've not looked for sirens to attach to an Avaya digital phone system, but I did have to look for a siren to attach to a different VOIP phone system. I couldn't find anything that was not analog. So... I ended up adding an analog adapter as another device, and attaching the siren to the analog adapter. I created a hunt group that had only those two devices logged in, and locked them in the logged in position. Changed the regular extension of the phone to something arbitrary and unpublished, changed the extension of the hunt group to what the original extension was, and changed the office phone's outgoing CID string to masquerade as the hunt group.
A caller would 'ring' the old extension, and the PBX would alert all devices in that hunt group, thereby causing the office phone to ring and the siren to sound (and in this case, the strobe lights to fire). IIRC, I had the hunt group forward to the voicemail extension that was associated with the office phone (the new extension -- not the extension that is now associated with the hunt group). Anyone wanting to retrieve voicemail from that mailbox had to know what the real mailbox number was if they tried to access it remotely, but other than that, the solution worked. That has been... 4 years ago, maybe. I posed my situation to several forums and newsgroups, and nobody had any better solutions for me. I can't say that it's the best solution, but it worked for me.
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