Sounds like a POS (point-of-sale) issue? I'm not sure finnair does a lot of it, but all airlines can control seat availability based on what "territory" is selling the tix. Did you call sales in the same country as the website?
Also, did you dummy-book the exact same itinerary on the site as over the phone? Finnair does 'married segments', ie are controlling availability based on connection. For example, You may very well see I class availability on LHR-HEL and on HEL-CAN separately, but none on LHR-HEL-CAN.