asterisk users - May 2005 - asterisk sip register with no username and password.

I am connecting to a local nortel PBX. The person setting up the PBX did 
not
define a user name and password (dont ask why). So I presume my register 
command
can leave off the username and password and look like:

[sip.conf]
register localpbx/5551212

I presume that is good enough then so calls coming in to 5551212 would get
routed over to my asterisk box?

Thanks

Jerry