OP I am with you. This is nonsensical and they will throw it anyway, it's not like they will give th écouter to someone else.

Have you now returned all or not?

If not, I would put it all in a package, with a copy of the contract and section highlighted and send it to them with a list of what you are sending mentioning that you consider the matter closed.

Might be just a few people at customer service that cannot do their jobs. Unfortunately those are everywhere, Swisscom has them too at times
I have just hanged up and called someone else.

In any case, stay strong, 25 CHF is not worth your sanity.