At my first attempt copying the entry from an old peer definition I was getting a "404 Not Found" message with the asterisk server not even trying to authenticate to the peer when I was trying to make a phone call. I thought it was a problem with my asterisk not wanting to authenticate but it wasn't. What should had happen is that the asterisk should have gotten a "401 Unauthorized" message, and then it would try to authenticate.
After reading a lot I came to the solution by myself comparing asterisk messages with a csipsimple client that I had running, my asterisk was saying something like...
While the csipsimple client has the server (IP or hostname) specified instead of anonymous.invalid. This first problem was solved with a fromdomain=whateveryourpeerexpects line on the peer definition.
So, I then got the 401 message and the asterisk was trying to authenticate, but this server was expeting to have an authuser on the registration (what is also called the digest username, ...) and even though on asterisk sip.conf doc there are examples on how to use authuser on "register" commands there is none explaining how to do that on a peer definition, I got to a lot of doc explaining how to do that with ways that people was saying that weren't working, patches for asterisk, ... I tested and tested and nothing worked.
I was almost going to go to bed again without fixing this (and it is about time 2AM already) when I started to test things by myself and found that it was defaultuser where I shoud specify this authuser, in fact I had tested this already but it was when I was having the 404 error, so it wouldn't work until I fixed that.
If you read the sip.conf doc, you'll find that:
;defaultuser=yourusername ; Authentication user for outbound proxies
which is quite clear and it's why I had tested it at first, but getting the 404 message made it not work, so in the end my peer looks like this: