Italian antitrust regulator AGCM has opened a probe into Telecom Italia SpA, Wind Tre SpA, Vodafone Italia, Fastweb SpA and industry group Assotelecomunicazioni over the telcos' suspected collusion in their 28-day billing practices that are deemed anticompetitive.

The watchdog said it seeks to determine whether the telecom operators "coordinated their commercial strategy" in relation to the frequency of renewals and billing of their fixed and mobile services.

In a statement, Telecom Italia "categorically" ruled out having coordinated with other operators as regards its billing practices. It said it has always complied with existing industry regulations and is in full collaboration with government authorities.

Fastweb also denied its participation in the alleged cartel, Reuters reported.

Italy's Democratic Party earlier moved in 2017 to ban 28-day bills, giving telcos 120 days to resume issuing monthly bills, Telecompaper reported. The operators committed to resuming billing charges on a monthly basis beginning in March and April.

However, consumer association UNC complained to AGCM and telecom watchdog AGCOM that the telecom operators unreasonably increased the subscription fees for their services to compensate for reintroducing the 12-month billing scheme, according to Telecompaper.

Vodafone Italia is a unit of Vodafone Group Plc. Swisscom AG owns Fastweb.