Billed for Cancelled Contracts

The most pathetic Service Provider I have ever dealt with. Cancelled my 2 contracts in July 2018 and paid the commitment amount for early cancellation. I received an email on the 13th August 2018 advising me that my query had been escalated to the Billing Department and someone would be in contact with me in 46 working hours...well I am still waiting. When I spoke to the Retentions Department on the 23rd August 2018 I was assured that my contracts had been cancelled and no further charges would be raised. Well he was wrong, because yesterday I received invoices for September 2018. I have just spent the last half an hour trying to get through to the Billing Department with absolutely no joy!