A common misconseption is that SIP registration process is primarly intended for peer authentication. But that is not the case. SIP registration process is primarly a mechanism through which an IP PBX (or any SIP server) receives information about a IP IP address of a SIP client (extension). Without that information, the IP PBX does not know where to send incomming calls.
Authentication part of the SIP register process is a good security mechanism that limits fraud options for an atacker. But even if a client successfully registers to a device, an attacket can still send a SIP Invite with a spoofed source IP address. The authentication process on SIP Invite messages is there just to avoid such an attack. If the mechanism is disabled, a SIP Invite with a spoofed source IP would be accepted by the SIP server. If a SIP server requires authentication on every SIP Invite, that limits the options for an atacker to take advantage of the VoIP system.