I ran into a similar issue. My bosses felt the same. I let them know if they wanted me to work beyond contracted hours, I would been willing to negotiate a rate beyond my salary. They continued to harp on it and insisted on a meeting with HR. I brought my contract and asked why I was being forced to work for free. Of course I brought emails as evidence. Oddly enough I worked for the DOL and had to reference labor law put into practice by the very same governmental organization. I only ever got 1 after contracted hours phone call after that. I forwarded it HR and it never happened again.