I can't see how signalling a phone would comply in any way shape or form.
There are a few firms that produce monitored transmitters / pagers - deaf alerter and Scope being two.
Scope also make various monitored boxes of tricks to put on desks etc
Unless you are going to install VADs all over the building (could do it wireless) I don't know of a simple solution .......
You're quite right it's not seen as a complete compliance but to be used in conjunction with beacons etc.
Great advice by Fishy, user experience is important, ask them. Pagers can do a good job, but they are often forgotten at home, in a desk drawer, not charged. Plus
We designed dms full well knowing it couldn't be a primary means but because it uses peoples existing mobile phones which they are glued to.
It's a really interesting case of guidance over user experience.
You want to look at areas where they aren't going to see everyone heading for the exit so toilets, kitchens. What are they more likely to take with them from their desk? Pager or their own phone?