Could be, I was a bit hasty. They offer as identification option mobile-id (certificate in your SIM), but my Finnish mobile number doesn't work. Also tried to log in with my Finnish bank credentials (they support SEB, Swedbank, Danske and Nordea), but even that failed. I guess I would need Estonian bank credentials...

Too bad, I have not planned an trip to Estonia in the near future and I think I will not bother with it just to get fourth Jolla. Estonian mobile-id would be interesting though, not sure if I can get one just like that without Estonian ID. Now I regret that I haven't moved forward with Estonian E-residency.