(Telecompaper) Italy's antitrust and competition authority, AGCM, has announced the launch of an investigation into possible collusion by the operators Telecom Italia (TIM), Wind Tre, Vodafone Italia and Fastweb over their controversial 28-day billing practices. The probe also includes industry group Assotelecomunicazioni (Asstel), with the antitrust watchdog already having carried out inspections in that regard at Asstel's Rome office. In a statement, AGCM said it's looking into whether the operators "coordinated their commercial strategy in relation to the frequency of renewals and billing of offers in the market of fixed and mobile telecommunications retail services following the introduction of new regulatory obligations."