Hello MichaelV,
Based on my experiences with 3 different carriers and several different systems, it mainly depends on the carrier and the system. I have customers on 3 different carriers. 2 of those carriers have no problem re-registering after a reboot with Samsung systems. One of the carriers we have to remote in and Disable/Enable to get it to register. The same carrier registers after a reboot on Panasonic and Vertical systems.