Probably obvious, but make sure your iOS version is up to date.
I have an iPhone 15 too. I haven’t experienced the OP’s issue, but a few months ago I had this really weird thing happen:
After I did some googling, I found that the ‘Apple CarPlay’ connection between the phone and the vehicle speakers can be hacked in that manner. So I don’t tthink my phone itself was hacked, just the wireless connection, but I updated my iOS version just in case and it hasn’t happened since.