Adding an additional telephone field to active directory schema?

We use an ASP script to generate a printable company phone list which pulls user and phone information from AD.  We use the "mobile" field in AD for the employee's company provided cell phone number.  We've been asked to add an additional field for employee's personal cell phone numbers.  We don't want to use the same mobile field because we need to be able to differentiate between a cell phone that is company provided and a cell phone which is the employee's personal cell.

I was thinking about using one of the currently unused telephone fields (pager) to store this information, but it would be great if we could create a new field to hold this information so it is properly labeled.  Is this doable?
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Yes schema extention is good practice. But its very dangerouse. I recomend try on test domain first.

This some manuals from MS

Note: new fields and attributes nor be shown in standart ADUC console. But ADUC may be extend too - VBScripts may be added to context menu 
FWestonAuthor Commented:
thanks, didnt realize there were so many caveats with this, i think i will stick with an already existing field
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