Still being billed for cancelled service

I continue to receive bills for business services (phone and internet) that were cancelled 6 months ago when we closed our physical office and transitioned to remote operations.  I have emailed the Account Manager assigned to my accout twice, including copies of the cancellation agreements, and have not gotten a response.  I have tried calling multiple times, only to sit on hold for so long I've had to hang up to address other priorities.  I am now receiving bills threatening to turn the account over to collections.  I am happy to pay anything that is owed through the termination date, but need to get the bills adjusted to remove charges after the termination date.  How can I get this rectified?