There are people's e-mail signatures that appear on the footer of their e-mail messages. Of course, there are also the multitudes of phone books maintained in Word or Excel by administrative staff company wide.
These varying mechanisms are all in conflict with each other and it is more likely than not that they are inconsistent and inaccurate.
So, what is the solution? In short, I believe in one single source of truth. You don't want all these lists clamouring to be the one to use in some situations but not others and just generally only serving a half purpose.
Populate your company address book. Even if you have to hand type it all in, do it. Ideally, use a smart bit of scripting or utilities like CSVDE, ADModify or LDIFDE.
Now, that's only the first part of the battle but it is the most important part.
You are now in the position where your roaming staff can suddenly, and reliably, use the lookup function on their BlackBerry, iPhone, iPad, Android phone or other device.
Show your people how to use this. Even if they knew the lookup function existed, now with one request they can retrieve a colleague's title, phone and fax number, mobile number, e-mail address and street address.
Similarly, your office users can use their e-mail client to find contact information. They don't have to search for the phone book on the Intranet, which will be out-of-date anyway. Time is saved, efficiency is gained, reliability and accuracy is increased.
That is elementary. What you are also going to show your company is you can now actually create the company phone book at will. This will be achieved by some basic scripting, whether on-demand (ie everytime someone opens the phone book document it is regenerated) or nightly.
Let's go deeper.