How to cancel a flight ticket with Iberia for free?

I purchased a flight ticket on iberia.com (not through a travel agency). Due to having found a train alternative, I no longer wish to fly this Iberia flight.

My ticket is non-refundable and I am ok with that. I want to cancel my ticket without refund.

The problem is that their website does not allow for this operation. Customers located in France (like me) are invited to call a non toll-free number. The cost of the call would not be dramatically high, but it's a matter of principle. I consider that if I cancel, they can sell my seat to someone else (having already pocketed my purchase, so the new sale is pure profit for them) so I don't see why I would give them any extra penny.

I could no-show, but it somewhat defeats the purpose of taking the train against the plane (the purpose is to limit my CO2 emissions). Indeed, they will be left with an empty seat that someone else could have bought, instead that person has to take another seat in another plane, and so on (I know it's not that simple, they have overbooking etc but that's the idea).

I have messaged them on Facebook Messenger but I don't think anyone checks this inbox, as my message was not delivered. I have found no email address or form to contact them, only their non-toll free number.

Are there options that I missed to cancel this flight for free? If I call the non-toll free number, what are the chances I actually get to have this flight cancelled? If I don't cancel it, is it better (the criterion being the fewer CO2 emissions, the better) to no-show and take the train, or to fly and carbon compensate?

EDIT: Problem solved through means proposed by accepted answer. However, I was able to apply this solution thanks to my phone plan, so this might not work for everyone. Therefore, if you have other ideas, feel free to write an answer.