Active Directory User Properties Updates?

I wrote a sub that would update an Active Directory user's phone numbers using the lines below:

SetProperty(dirEntryResults, "telephoneNumber", strPhoneNumber)
SetProperty(dirEntryResults, "mobile", strPhoneMobileNumber)

The telephone number updates successfully but the mobile number never updates although the strPhoneMobileNumber variable has a valid phone number value. The code executes without any errors. What do I need to do to change the mobile number?

Sub UpdateUserAcct(ByVal userLogin As String, ByVal strPhoneNumber As String, ByVal strPhoneMobileNumber as String)
Dim dirUser As DirectoryEntry = GetDirectoryEntry()
Dim dirUserSearcher As DirectorySearcher = New DirectorySearcher(dirUser)

[code]....