Powershell: reset link between custom attribute and AD?

We found a script that appeared to run great the first time (see the code at the end) . But now after several weeks of repeated runs, it longer appears to function, or something else is amiss. (when I run it by hand I see it change new accounts, but subsequent runs show no changes)

While I saw a specific user had the correct info after the first or second run, several weeks later that same user no longer has the correct info in the custom field. (its blank) many more users I look at display the same behaviors.  Most users *DO* display the correct info. (I have not tested to see if I change the AD field value whether the exchange custom attribute changes)

If I look at the ipphone field in the AD account(s), the proper data is displayed. If I go into the mailbox properties and go to General, custom attributes the field is blank. Yet repeated runs of the script do not yield any results. (I suspect this is normal if its simply linking attributes to AD fields)

I guess my first question is this a "hotlink" where the custom attribute should link and change automatically as the AD field changes, or does it simply copy the AD field value once? 

Can anyone suggest code that might "unlink" the custom attribute and "reset" the link so that I could try re-running the script below and "shake the etch a sketch" to get it back to normal?

Any other suggestions to fix the issue a different way?

Start script

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# Script to copy the IpPhone attribute the extensionAttribute1 attribute
# Written by Ben Lye - 07 December 2009

# Find the users in AD using an ADSI search method
$searcher = new-object DirectoryServices.DirectorySearcher([ADSI]"")

# Filter for enabled user accounts with a value in the IpPhone attribute
$searcher.filter = "(&(IpPhone=*)(objectCategory=person)(!(useraccountcontrol:1.2.840.113556.1.4.803:=2)))"

# Return the sorted results
$objects = $searcher.findall() | Sort-Object -Property cn

# Loop through all the objects that the search returned
ForEach ($object in $objects) {

       # Store some attribute values into variables
       $ipphone = $object.properties.ipphone
       $extensionattribute1 = $object.properties.extensionattribute1
       $dn = $object.properties.distinguishedname
       $adspath = $object.properties.adspath

       # If IpPhone is not equal to extensionAttribute1 then process this object
       If ($ipphone -ne $extensionattribute1) {
              # Get the ADSI object
              $adsiobject = [ADSI]"$adspath"

              # Set the attribute
              $adsiobject.extensionattribute1 = $ipphone

              # Commit the changes
              $adsiobject.SetInfo()

              # Output what just changed
              Write-Host $dn ":" $extensionAttribute1 "-->" $IpPhone
       }
}