Narong Juiset, director of Lampang Correction Institution for Drug Rehabilitation said, the prisoner was sent to a doctor after he developed a stomach ache.
An x-ray revealed he had what appeared to be a mobile phone inside his stomach. He underwent an operation to remove it, and it proved to be a small Samsung mobile.
Mr Narong said searches had since resulted in the seizure of 12 mobile phones that were being hidden in this life-threatening fashion.
It was extremely dangerous as the phone could obstruct the digestive system and cause death.
Mr Narong said the phones were smuggled into the prison by visitors, and the concealed by the prisoners.
More stringent searches will be made of items carried in by visitors, to prevent further smuggling of illegal items to inmates, he said
The Lampang correction institute is the largest in the North, holding more than 1,000 male inmates on drug-related cases.