Can anyone help me figure out why I am unable to edit the Dnsserver1 value?
Because you haven't unchecked both boxes to the right of the field you want to edit.
Because you made the same mistake as many other novice users (so don't feel bad): you need to remove both checkmarks from the right side of that setting, in order to be able to edit the value (this applies to every setting, not just DNS).
By the way, if you think changing DNS will fix the Google Voice issue, it won't.
Read: http://www.obitalk.com/forum/index.php?topic=13113.msg84139#msg84139 (http://www.obitalk.com/forum/index.php?topic=13113.msg84139#msg84139)