iPhone 15 Pro Max won't pair with older Bluetooth device

This was successfully resolved after calling Apple, it had to be escalated to one of their upper support tiers. The fix was to have an iCloud backup available, then restore the phone to factory settings. Once done, the Bluetooth device connected successfully. After that, the backup was restored. I don't recommend doing this without having an Apple support person walk through it with you. There are many opportunities to mess up. There are a few side effects at losing some application data. The major one is I can't see my phone's favorites list and the Outlook contacts went into my phone's contact list.