You could do...

Code:

$user.telephonenumber = "(000) 123-"+$oxl.cells($row,4).value
$user.setinfo

The reason it doesn't work in KiX is that for the Put method, you have to put the parameters in parens.

Code:

$user.Put("telephoneNumber", "(000) 123-"+$oxl.cells($row,4).value)
$user.SetInfo