Or send an actual letter in the post to the customer service department. I guess approximately nobody does this nowadays but it works very well for me.
Registered mail with proof of delivery works and is scary for them because it's a legally-admissible paper trail and proof of you trying to resolve the problem in good faith, which will complicate any of their attempts to collect money out of you down the line should you choose to stop paying (which you should also do if they don't address the issue in a reasonable timeframe).
Send the letter via overnight mail - or better yet, FedEx. Mail goes to a mail room which may or may not be screened. FedEx seems far more “important”, and make it to the executive’s assistant’s desk who is far more likely to act on it.
