Correct - you should be using PJsip. Also as far as changing extensions, you can run into the same issue even if you use the same extension number but you change the channel driver from SIP to PJSIP. This is how I've accomplished the task for remote phones that you can't touch. From your EPM, issue a reboot. Then real quick, you change the extension to use PJSIP, go to EPM, and then do a rebuild. If you do it properly, the phone will then grab the new settings and reconfigure itself and work just fine. You can of course do the same thing if you are changing the extension number on it.