Soooo, it is pretty clear now that the protagonist is insane and just imagining most of this shit, right?
First of all, the phone is shown from multiple angles and has not landline connection or power.
Then he recalls quite literally the couple which he expected to be in this hut.
Then, for a sanity check, all he basically says to the 'police' on the phone is: 'I am lost. I want to leave this island, but the only inhabitants here won't help me. I need medical assistance, because I don't know how I got here.' And as a response he gets the questions which stumbled him for answers from previous conversations i.e. 'In what way are they strange', 'Do you know how many unnamed islands there are in this country'.