I found a wired handsfree set in the local Prisma hypermarket: the Philips SHE3595BL. I was surprised by their quality that rivals more expensive Sennheisers, although Philips probably has a bit less bass. They are compatible with Jolla. And cost less than 20 euros! So these are great starter headphones in my view.

Jolla now supports the wired headset button, but for answering calls the Phone app must be open in background. I hope it could be made to work without the need to keep Phone app open, because I'll always forget to do that.