I have finally solved the problem. Reading the list I found sofia log level is controlled independently (I should learn this before!) and after reading the sofia log the problem was quite obvious.

The problem was that Kamailio was trying to authorize all the requests with its domain in the From header. Thereby, it was answering FS a "407 Proxy Authentication Required" and hence the FS auth request event. I have already solved it disabling authorization for the FS IP in Kamailio.

Regards.