According to this, Tiscali is blocking online play of Call of Duty 4 (CoD4) during peak times. It seems the problem's being cause by the ISP severing P2P connections during busy periods. CoD4 and other online titles use these connections for game-matching, rendering online play impossible. That report relates to the PS3 version of the game.
Whoops. Said one disgruntled user, "If they do not sort this problem out, they are on the verge of losing a new customer that only joined them three days ago. Tiscali sort the problem out or lose a lot of business. Simple choice really."
Tiscali has made no official statement on the matter.