Thank you so much for a comprehensive answer. I will get back to you as soon as I can. I will now first try a third solution.
To my great surprise, the plugin played right out of the box. That is, in Vipp's test environment. Vipps delivers test api's, a test merchant, and a fake phone number, a fallback url and even some money to try out the payment before going live.
Everything worked, the payment triggered mails with download links. And best of all, not Hikashop, not Joomla, but Vipps asked the customer for the phone number. Brilliant. I changed the fallback to my own, and still everything worked, so I put the production keys in the plugin and went live.
Result: nothing worked. No email, just an error message that Vipps required phone number before checkout.
But I can not forget that Vipps asked the customer for the phone number in the test environment. Is it possible to achieve the same live?
I have sent requests to the plugin creator and to the Vipps crew, but they are not as fast as you in answering. Let's wait a little longer.
Regards
Svein-Erik