So.. 'Auto Bluetooth' did nothing at all. So I scrapped that and tried 'Bluetooth Auto Connect' which to be fair to the developers, comes with various FAQs about different options to try to get a result and that they couldn't guarantee anything. I didn't get one. The best I could force it to do was to auto-reconnect every time it disconnected, which made it 'ping' every 10 secs or so as it cycled through the connect-disconnect routine and still dropped calls midway. The damn things just don't want to talk to each other.
Am beginning to resign myself to accepting that the Nissan BT adaptor simply does not support some newer phones (see attached). Unless anyone knows how to change the BT adaptor pin code...??
NissanBTAdapter_Compabilitylist.pdf