Bluetooth old Iphone SE to Amundsen in Yeti for hands free

2017 Yeti with Amundsen

I just want to pair an old iphone SE for hands free phone calls, nothing else.  But the phone doesn't seem to recognise Amundsen when it's looking for a Bluetooth pairing. Amundsen can see the phone (or at least it sees a phone which I assume is mine) but when I try to pair, it says the phone is not supported. I suppose the phone may be too old, or am I doing something wrong?

Step by step explanation for a technophobe would be much appreciated!