General idea for PBX stuff:
People have multiple phone numbers. Use your PBX extensions cleverly to provide speed dials to their home/cell phones (if appropriate).
if your extensions are all 2xxx then you could have:
!!! 2010 -- Joe's office extension (coz Joe is often in the office) 2011 -- Joe's Extension at the office 2012 -- Joe's work Cellphone number 2013 -- Joe's Home phone number (unless of course you shouldn't be calling Joe at home) 2014 -- Joe's personal cellphone 2015 -- Nearest fax to joe 2016 -- Joe's voice mail 2020 -- Bob's work cell 2021 -- Bob's extension at the office 2022 -- Bob's work cell number 2023 -- Bob's home cell number 2024 -- Bob's home phone number 2025 -- nearest fax to Bob 2026 -- Bobs Voicemail
and so on. So 2xx0 is the preferred way to get hold of someone, and the number you will normally call to get hold of them. If you want to call them on their work cell then you do "2xx2", and this will work no matter who you're calling. If you desperately need to get hold of someone you can startat "2xx0" and work upwards, getting "no such number" if they don't have one of those, or they don't want people at work to have it. Bob can dial 2026, "*" or whatever and his pin to check his email from anywhere.
Don't group people by where they work. Generally you don't care where they physically are, if you're phoning them then they aren't close enough, and anyway chances they'll eventually change their location. If you are going to group by anything, possibly group by timezone so you know if you call someone with "21xx" then you're potentially calling someone who won't be awake.
111 or 911 should always get emergency services without having to dial an outside line. This could save someones life!
In Hamilton all local calls start with "8" or "9", and all toll calls start with "0". So if you just send all numbers that start "8" "9" or "0" to the PSTN without modifications you don't need the pesky "dial 1 to get an outside line". Leave "1" as a prefix for get an outside line as there are some possible numbers you might want to dial (eg 123) and catch "111" to send to emergency services.
If you don't do this, the poor saps who have extensions that begin with 8 will get a lot of interruptions as people accidentally call them. 0 to get an operator means the operator just gets all the people trying to make toll calls too.
If you're using ENUM, consider publishing any DDI's with 
e164.org. It's free, and means other people can call you over the Internet without touching your expensive DDI's.
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