No auth_challenge use Proxy-Authorization for INVITE messages but we need to use WWW-Authorization. A possible solution would be use [modf](https://kamailio.org/docs/tutorials/5.3.x/kamailio-kemi-framework/kemimods/#...)
I do not understand why this peace of code
``` KSR.x.modf("www_challenge", realm, 1); ```
gives error
``` app_lua [app_lua_api.c:1725]: sr_kemi_lua_modf(): function 'www_challenge' has fixup - cannot be used ```