I personally not set any sip user extensions in sip.conf because from the beginning, I use GUI to set up the users. Then I all along work with the users.conf to add users manually. This may be one of the reasons that I need to use Dial(Local/6000) instead of Dial(SIP/6000) to dial within dialplan.
I will try further.
Please note template function is very useful for creating multiple users of similar capability.