Thanks everyone for your replies!
We do have resiliency including ISDN (via resilient framer), but in almost all setup, both systems are in the same power source. This system that's ready for activation, happen to host over 200 users.
With just 100 users, I've seen the phones download the flash firmware twice, then take close to 5 minutes for the download.
So yes, it will be an outage which management won't agree on
.