Of which phone do you speak? If a SIP set, or ATA reboots It is probably not necessary to "unregistered" in most cases bacause it will be re-registering again in a very short time. There may be some cases, with some providers or PBX's that are kept "happy" if there is an unregistered. Some sets will also be re-downloading a configuration, so that may have something to do with it. I would believe that in most cases, it doesn't matter.