WorkPhone Remembering Old Values

There is something strange happening with our User Profile syncing with AD, specifically with the WorkPhone property.  I have a staff directory webpart that is displaying our staff members, names, titles, departments, email and Extensions from AD.  In the CA, I mapped the WorkPhone property to the ipPhone property in AD.  It is displaying mostly everyone's phone extensions properly (a four digit extension, ex. 3347).  But there are some staff members whose Extensions are showing as a full telephone number like this: 5617987600.  So I went into each of their User Profiles and deleted the phone number to leave just a blank field.  I ran a full sync and they were gone, which is what I wanted because each of these people do not have a personal extension.  However, after the next sync that happened, all of those full phone numbers came back again for those people.  Where are these numbers coming from?  They are not in AD.  And how can I get those people who don't have a phone extension to just appear blank?

Thanks,

Kathy