What you said
You are going to have to enlist the parents. Maybe start with the worst offenders. Call and say something like: "Hello. I'm Ryan's electronics teacher and I am calling to introduce myself. I wanted to let you know that I think Ryan has a lot of potential and I can see with the right help he will progress. I am calling you since I need everyone to catch up with X."
If you hate the phone, write yourself a little script and read it to the parents. Itemise the behaviour, as most parents won't understand "school- speak". Also divide and rule - play musical seating plans. Pupils hate not being beside their mates. Use praise where it is due.
The expert view
You can nail this. The fact that you were "The Guy" in your previous incarnation tells me everything I need to know.
You have only just taken over a class full of institutionalised mucking about and lack of interest. No wonder it is tough. It is not your fault; it is the situation in which you find yourself.
You laid down new rules and they do not like it. So far, so predictable. Now what you need to do is keep it up. Set detentions, track detentions and repeat. If they do not attend, escalate and enlist help from line management. Children are pretty simple beasts at times. They crumble in the face of consistent resistance. It is when you stop persisting that they realise they can run rings around you. So keep it up; do not waver for one second.
If you have a problem with phone calls, there are ways around it. Letters home can serve just as well. How about emails? Or enlisting a colleague to help?
Tom Bennett's latest book, Teacher, is out now, published by Continuum.