Do these headphones have a mic? If for any reason it thinks they do when they don't that would be the result.
However, that requires either a faulty socket, a strange sort of short, a very strange software corruption or both pairs of headphones being broken in the same way. I think we can exclude the last. Did this just start, or was the phone always like this? If it just started, did anything happen before then that might be relevant (dropped, system update, has it ever got wet?)? And just to be sure, if you shine a torch into the socket can you see anything in there? If so be very careful trying to remove it, as you could easily do damage in the process.