So, do you set up your extension under GUI or just paste the codes into sip.conf? I believe I can't use SIP/6000 is due to the setup under GUI. The information is in users.conf not sip.conf.
My 6000 is assigned to the Siemens IP phone. Works without problem.
Only I did not put any extension setting in SIP.conf.
My point is since we need to add the extensions setting in sip.conf and iax.conf. What is the point of using GUI to create users/extensions? Only you can see them under GUI.