First set Ticket::Service::Default::UnknownCustomer to YES
http://your.otrs.host//otrs/index.pl?Action=AdminSysConfig;Subaction=Edit;SysConfigSubGroup=Core%3A%3ATicket;SysConfigGroup=Ticket

and edit default service:
http://your.otrs.host/otrs/index.pl?Action=AdminCustomerUserService

I hope this will help you.

-N