From HiPBX Wiki

Providing reliable power is the first step to a Five Nines system, and is actually the easiest part. Everyone has a UPS these days, and most of the hard work should already be done. But don't just plug it all in and don't think about it. Look at this layout for an example.

You will see that everything has a feed from both the UPS and from the GPO. This ensures that even if the UPS fails, you can still swap it out without an outage. And when the GPO goes out, everything will be running on one PSU, being fed soley from the UPS. Having hot-swappable power supplies on your servers and devices will also ensure that if a power surge gets through your surge supressor, you can replace PSU's without bringing too many things down.

You should also note that EVERYTHING is duplicated. You can break any one thing on that image, and everything will keep working. If you have dual power supplies in your switches (which would be better, but tends to be a LOT more expensive), that would be set the same way as the other devices. I was going to have switches with dual power supplies in the diagram, but it's much more likely that you'll be using single PSU switches.

As an important aside, when you are organising and sizing the UPS, ask the client 'How long do you expect your phone system to run without power?'. Double the amount they say (unless it's already a reasonable amount - I would suggest speccing it for at least 2 hours, preferrably 4 hours). It's quite often that they don't even realise that '15 minutes' is barely enough time to do anything. If they say 'indefinately', then you'll need to start organsing generators and other fun stuff. Luckily, this is an easily solved problem - you just need to throw money at it until the problem goes away.