As another user mentioned, I think the safest way to do this in the scenario you described would be to establish a code where innocous sentences have a certain meaning attached to them in advance, but that of course would require the hypothetical person to be prepared/expecting imprisonment. Another thing I can think of would be smuggling a small phone in the ass or another orifice, but that would be a lot more risky, as it’d be likely that they’d check every square millimeter of the person’s body and being caught probably wouldn’t end well.