Active Directory - User Telephone Number copy to ipPhone Atttribute

I am trying to copy the last 4 characters of the phone number with the following scenario, but I get an error. Help please :(

Error :

You cannot call a method on a null-valued expression.
At line:3 char:5
+     $phoneNumber = $_.OfficePhone.Substring($_.OfficePhone.Length - 4,4)

Get-ADUser -Filter * -SearchBase 'OU=Test 1,DC=domain,DC=com' -Properties OfficePhone | ForEach {

    $phoneNumber = $_.OfficePhone.Substring($_.OfficePhone.Length - 4, 4)

    Set-ADUser -Identity $_.SamAccountName -Replace @{ipPhone=$phoneNumber}

}

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