TNo word from a family member backpacking across Europe?Urgent message for friends touring East Asia, address unknown? America Calling, a new overseas message service, will print one minute phone messages in code in the "International Herald Tribune," available daily from Paris to Hong Kong. The basic service is $29.50. For details, contact America Calling, 2406 18th Ave., N.W., Olympia, WA. 95802 (202-786-888 8).