Zwanzigerrufen (XX-rufen) is an Austrian Tarock game for four players, somewhat simpler than Königsrufen, and played with just 40 cards of the Tarock pack. The name means "Call the XX", because you call the tarock-XX to determine who will be your partner.
You can find the rules on this archive copy of Bernhard Krüpl's Tarock Page.
Players in North America can obtain Austrian Tarock cards from TaroBear's Lair.
You can play XX-rufen on line against live opponents at the Ganetwist site (the interface is in German).
The German site Skill 7 includes an online XXer-Rufen game.
A XX-rufen computer program is available from KEC Austria: Triangle productions.