3.5mm Jack Plug does not fit my iphone 6s plus?

I've inherited my brother's phone, it's a couple of years old. He never really used it...just sat in the box. Thing is, I cannot find a jack plug to fit. I need it to attach an extension cable for speakers, not a headset. I had a 6s and my plugs fitted perfectly - a nice solid click when they went in. My headset is a Betron and its jack plug fits, but it doesn't feel solid...just like it's squeezed in.

I've checked the socket and it is perfectly clean.

Any thoughts anyone?

David

ps. I put this into accessories because I couldn't find a better topic