Here, hotels have deals with booking.com etc that forbid them to post lower rates on their own websites. However, if you call to reserve, you get a discount code. I don’t know if that’s a more global practice or not.
~5 years ago the EU was starting to push back on the legality of the MFN clauses and I was under the impression that Booking.com had scaled back their usage of them in some ways, but I don't know the current state of things.
While I'm at it, my usual rant about Booking.com's review system - the scale is actually from 2.5-10, and the median score is 8.1. Don't be fooled by those "above average" 7 scores.
Over the year I have learned to overcome my aversion to calls when making one is beneficial, but a call to someone totally unknown, and not in my native language, is one of the cases where... yeah, I'd pay the premium too.
What if another hotel was willing to go lower ?