I'd like doing test using authentication if possible... uac.xml from those examples does not cover this :(
You should read a little more about SIPp before using it. The documentation is concise and clear: http://sipp.sourceforge.net/doc/reference.html You will get faster results if you don't have to wait for responses for simple questions like this (and actually, this is not the proper forum for the inquiry).
I have answered recently a question in the sipp-users forum about this same problem (server responding with 407 all the time). You might want to take a look at the discussion as I have provided a corrected version of an auth scenario and the OP said it was working so you could adapt it to your needs: