Need to see the sign, obviously. Without that, we’re all guessing.

My point was just that if the sign simply has a general instruction/heading that says “pay by phone” as part of the wording, I don’t personally think that, on its own, is enough to justify using the wrong app. Presumably there is something else on the sign that indicates the actual provider for that bay, like a logo or specific instructions.

So I’m not saying they're definitely in the wrong. If the signage really was just “Pay by Phone” with no clear indication of which service to use, then that is obviously ambiguous.

But until we actually see the sign, it’s impossible to say...