Let's hope that the sales office just hasn't gotten notified of your rescission. Did you tell the caller that you rescinded within the allowable timeframe and are just awaiting refund of your down payment? Did you tell them that you have proof of mailing from the post office?
Knowing Wastegate as we do, it would come as no surprise to have a salesweasel call to try to save the sale. We've heard of them calling and telling people that their reason for rescinding 'isn't good enough' (no reason is asked or expected). They may just preying on your anticipation of the refund. They have up to 45 days for the refund to get to your credit card, yet no 'progress report' on the rescission is required.
Assuming you have the necessary proof that you sent the letter in time, just don't answer any calls from them. Let the salesweasel sweat, not you. The letter will work it's magic, and nothing said on the phone is binding.
Jim