Outlook 2010 Phone number field

I have an Outlook 2010 user Jsmith@example.com.

 on the  Address Book, the phone number will show as correct ,ex: 999-999-9999
however if himself or any other Outlook 2010 user will open new email type jsmith in the To: field then click "check name", the full name will be displayed on the To: field, however if you highlight the full name and hover the mouse over it, the little popup will show a different phone number,ex :888-888-8888

This happen if only you do it from Outlook 2010. if you do it from Outlook 2013 or Outlook WebApp, the correct phone number will show up after you highlight and hover the mouse over the name .

I checked if user has contact locally that might have that wrong phone number but could not find it..
I have not checked if there is contact in ACtive DIrectory, but there is an AD account for the user with correct phone number, that 's where Address book pulls it from.

I would like to know where that wrong phone number is coming from.

Thank you