I work at a broadband company, the technical support department. I finish at 5 but if I'm on call with a customer, it can go over 5. We were told we are paid in 15 minute increments for overtime. I told my boss throughout the month when I worked extra. For context, this company is very new and not very well ran. They constantly underpay and most of the time, they have just not tracked anything properly. Now this month I received 100 pound less for my overtime than what I was entitled to. I told my manager 2 months ago that if they don't pay me for the 15, 30 or 45 minutes of overtime I do, then I will refuse to stay after 5 in future. He nodded and gave the usual 'of course, it's your money, you gotta make sure you get what you deserve.' Now I've been underpaid, could I just tell a customer at 5 to 5, 'sorry I've got to end the call, I'm going home'. Like, could I get in trouble with work?