If you have been playing with 12/12 versions of FreePBX/Asterisk, then you may have noticed that registering a SIP extension is not quite as straight forward as it once was. In advanced settings, there is a drop down menu for "SIP Channel Driver" which allows you to choose the new chan_pjsip driver, the old chan_sip driver or both drivers. If both are selected, then FreePBX offers both extension types when creating a new extension. In my initial testing I never got a SIP registration to work, only PJSIP.
This post over on the FreePBX forum explains why, each service is bound to a different port. By default in PIAF Black, PJSIP registrations are accepted on 5060 and SIP registrations are accepted on 5061. When configuring extensions to register via SIP, you must specify the port number.