Thanks. I had already tried that website and failed hence the message. Given your message and nice link to Cockpit, I still can't do it, because I am not a Swisscom customer, and am overseas, so I can't connect to their mobile network.
"Unknown number. Access is only possible with a NATEL® subscription." No idea what a NATEL subscription is.
I just want to pay £30 in advance, without any other subscription with Swisscom, for an eSIM for a given phone number (that of my son).
We are all overseas until arriving and needing to use the eSIM, without being able to go to a shop in Switzerland, as he will be with a school group.
Finally, I am doing this not on my son's phone, but on a laptop. It would be possible but a bit fussy to connect to the phone's mobile network. I hope they make this easy.
Thanks again, Rufratzusch94