There are many schools of thought out there on dial plan provisioning. Obviously you need to avoid any situation where you will have overlapping numbers. You also need to provision your dial plan for growth.
The first step is to complete review of your current dial plan. Next, decide if you have any sites with overlapping did's. If you only have "one site" of 1000 users you could get away with a 4 digit dialing plan. However, as the number of sites grow, the likelihood that you will run into overlapping did's increases.
I have come to like a 10 digit dialing plan for "inter"office calling and 4 digit dialing for "intra"office dialing. This does a few things:
1. It removes the possibility of overlapping dn's.
2. Allows you to keep your 4 digit dialing in an office.
3. If you are currently in a tdm environemt and don't have your pbx's/key systems networked together your users are used to dialing 10 digits to other sites anyways so you will not have to re-train their dialing habits. (if you used "site codes" your users would have to be trained on using these sites codes for dialing which may or may not be a good thing).
The main drawback to this approach is in Unity. Unity does not currently have the dial plan scalability that callmanager does. Basically you cannot partition off sites as it relates to your dial plan. In Unity a DN can only be used once. Therefore, with the 10 digit dial plan in callmanager, users calling into check voicemail NOT from their phone would have to key in a 10 digit number. You can however utilize alternate extensions in Unity so people can program unity to recognize their cell phone for example and they would not be prompted to enter their 10 digit extensions, just their unity pin.
The SRND guide as a section on dial plan as well:
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_implementation_design_guide_chapter09186a0080447504.html
andy - berbee
