Jakob I like your thinking :)
A call centre is tricky. You have to be pretty sure that you have a legitimate user on the phone, such verification is one of the hardest things to do. bunq also can't provide a 24x7 call centre themselves, so you have to outsource it, that on itself can be risky too. And this also sounds like an expensive solution for a relatively small problem.
A web interface just to block or freeze a card can be a solution, but also in this case you have to be sure that the right person is asking for a block. You could ask for specific things like email, passcode and so on. But this would also be attractive for misuse. Before you know it, there will appear phishing sites of other tricks to retrieve this useful information from you. That is one of the reasons why bunq is mobile- only.
You could think of an automatic phone service where you enter your unique code and a card number, and it will automatically get blocked or frozen.
This is less tricky than an online service. But users can't remember such number you might think? Well, every smart traveller will always write down important numbers and never fully trust their phone.
Another option is the most simple one: bunq will be liable for all the losses that occur between the moment you reported your card/phone missing and the moment you reasonably will be able to contact support.
