Charged for checking if the device work

I was called by the operators saying they need to do check up or service on the device after I was hijacked to check if it wasn't tempered with and agreed for them to do it. And the technician came and checked the device and all was good. Now I get a call saying I'm arrears as they billed me for the technician ans i was not told that I need to pay for the checking the device. If I was informed then I was decide if I agree or not depending on my budget. This is such a foresight from them and I'm not happy at all.