The chief executive officers of Safaricom, Telekom Kenya, Essar Telecom Kenya and Airtel Kenya later denied there are active SIM cards which are unregistered.
The mobile operators said that since the publication of SIM registration regulations, all four operators "have gone over and above what is required by law."
"We wish to inform the general public that since the publication of the SIM registration regulations in January of 2013, the mobile operators continue to maintain and update a registered subscriber base of over 30 million mobile phone users," the mobile operators said in a joint statement.
Vodafone PLC is a substantial shareholder in Safaricom, which is publicly listed. France Telecom has majority shareholding of Telekom Kenya, and Essar Telecom Kenya is owned by the Essar Group and run by the brand yuMobile in Kenya.
In 2010 Kenya made it mandatory for all SIM cards to be registered following an upsurge of phone-related crimes. At the time kidnappers were receiving ransoms through mobile money transfer services.