If we want the private ID, then that's in the Authorization header - if we don't have that, then we don't have a private ID, and so we return false to indicate that.

RFC 3261, section 10.2 on constructing REGISTER requests:

      To: The To header field contains the address of record whose
           registration is to be created, queried, or modified.  The To
           header field and the Request-URI field typically differ, as
           the former contains a user name.  This address-of-record MUST
           be a SIP URI or SIPS URI.

Note that the private ID is *not* a SIP URI and therefore cannot be in the To header.