Netstar told me they needed to install a free back up device on my car but then they actually installed a second device and charged me twice the agreed monthly amount

Netstar told me they needed to install a free back up device on my car but then they actually installed a second device and charged me twice the agreed monthly amount. I've been calling for week to try and get it sorted and all they do is transfer me to different departments but never and none of them help me. They also keep calling to sell me other products but they don't call about my problem. The product works fine and I have no complaints but the customer service is not good at all