The Apostle Paul wasn't a "prisoner in a jail cell. He was a prisoner in the Lord (Eph. 4:1)." In Acts 16, the Lord was in the jail cell with him to reach the other prisoners and the Jailer and his household. Paul was singing. We may blame our outward circumstances for our dissatisfaction, but people become dissatisfied for every reason and for no reason, even if they have freedom. I have people around me who are chronically dissatisfied. The source of their dissatisfaction is within themselves, not their environment. I also know people in communist China, who are full of joy. Paul said that he learned the secret so that he could be content in whatever circumstances he was in (Phil. 4:11+).