If you register a new handset, and you do not assign to a SIP account , outgoing calls would work, but you will have troubles about incoming calls, as you will get 604 SIP error regarding this.
You could check via xml provisioning file if you have :
looking into http://IPADDRESS/Settings.html?xml=1
So entering into extensions web interface, just assign that account to an headset IDX (number) . Enable checkbox and select Save option as follows.