Poor service

Paying for a 20mbps line and only receive 3mbps. Tried to resolve this for 4 months without any success. Phoned then again requested to fix and will not pay before the line speed is rectified. Vox suspended my line and said they will only try to rectify it once payment is made but for four months they could get it to work for what i paid for. Keep on *****ing money without providing the service according to the contract. Also wont allow you speak to a manager or a owner.