Can't cancel service / No response

We want to cancel one of the phone numbers on our contract, 2 products on the other number as well as the debit order. I have been trying since my first email on 12/11/2014. On 19/11/2014 I sent the necessary info they asked for in their response email on 18/11/2014. Since then I have resent the email on 12/01/2015. I also phoned them in December/Jan and they said they'll come back to me. I tried to phone them again today (Cancellations: ***) but after holding on for 20 minutes, I am now fed up and trying different measures! I can't just cancel the debit order and refuse to pay, because they'll just suspend the service and the one number is our business number and we can't afford it to be suspended!!! WHAT MUST ONE DO TO GET DECENT SERVICE!!!