The police have seized a mobile phone from a prisoner in the Central Jail at Thorapadi here.
Sources said prisoners had been using mobile phones to be in touch with their associates and some of them continued their criminal activities.
Following this, surveillance was tightened in the prison premises.
Special inspections
A few months ago, a special team inspected the prison and ordered action to increase security.
On Friday night, prison staff on patrol found a convict, Subash alias Reuben, 53 using a mobile phone and seized it. He was involved in a drug abuse case.
In 2012, he was arrested in a drug abuse case and shifted from the Puzhal prison in January.
The police are probing the source of the mobile phones and how they are being smuggled into the prison premises.