I assume this is the problem mynumber@neofon.tp.pl@neofon.tp.pl
In the username/SIP umber field you should just have the number (mynumber) and not the IP/Host name (@neofon.tp.pl), otherwise Express Talk will try to register username "mynumber@nefon.tp.pl" with the "neofon.tp.pl" when your username is only "mynumber"