FWIW, my company recently switched the whole firm over to IP telephones to combine the phones and data into one data stream to manage.

Two real world observations/experiences:

1. They installed a back-up, traditional self-powered phone to one phone line and labeled it "for emergency only." There is no back-up power in our office if the main power goes out.

2. I had to make a 911 call for police assistance because an intruder had gained access to a restricted area after hours. Another co-worker and I were still in the building. I called from the IP phone at my desk, not the emergency phone because I would have been seen by the intruder if I had gone to the emergency phone. We had been told that the 911 call from the IP phones was properly programmed and tested. The 911 call went through OK, except it went to the wrong jurisdiction! They were able to connect me to the right jurisdiction, but it still involved a delay. Don't know what I would have done it they had not; I guess try the operator. Police arrived and problem solved.

This leads me to be very wary of IP phones where the system does not have back-up/UPS power and where some programmer somewhere decides where the 911 call emerges from the network.

Both fixable problems, but, I rather it not occur on my watch.