This is just a quick observation. You might be tempted, for the sake a clarity, to create an additional field next to the Contact phone number to capture a phone extension.
Even though it might seem like a very good idea, after all a lot of companies do still use phone extensions for each employee, when users will synchronize with Outlook, this field will of course not be mapped to anything in Outlook. Obviously, it’s a custom field, so it makes sense.
Anyway, the only way to store an extension and have it mapped between CRM and Outlook is to mark it in the phone number field. Try to use a convention across the company, so you can handle that in a formatting script.
I am still surprised at how many people still mark extensions with the following format:
(999) 999-9999 x999
(999) 999-9999 ext999
One note, for the sake of telephony, if you use a comma [,] instead of the [x], the cell phones will know to interpret that as a pause, and, once dialed, should be able to dial the number, followed by a pause and then the extension.
Make it easy for the mobile users to dial phone numbers!