Z., the role of the billing department, whether a global service for many medical practices, or one employed "in house", is to send the bills correctly to the insurance companies or patients, & to collect the amount due in reimbursement.
However, the responsibility for making sure the bills are accurately charged falls on the individual departments. Sometimes, this is overseen by medical records and compliance departments, and sometimes it falls directly on the providers of service, such as in your case with your therapist.
All services provided need to be documented, so a billing department cannot arbitrarily remove a charge simply because a patient calls in & says to do so. Once they receive verification from the therapist, it will likely be taken off and the amount you owe will be adjusted.
I'm not sure why the billing department is requiring that -you- speak to the therapist. When I worked in the business office of a hospital group previously, we would send communication to the departments or medical records, and adjust claims accordingly. Hopefully, once you speak to your therapist, this issue will be resovled.
Definitely follow Diane D's advice, & keep a log of who you call, when & what was communicated. Start that now, & include the original communication to cancel the appointment.
Best of luck! T.