Since, to the phone, there's no difference except the code of the character, but the site can have a box that ignores anything but digits, or just about anything else, I'd guess it was a problem with some of the mobile versions of the sites you're visiting. Have you tried changing the web browser to get the desktop version of the site? (You may have to do a lot of "unpinching" to get it large enough to see a box to type into.) If that works, it's the site.
(Don't put it past a lot of "web developers" to have used the same plugin to get a mobile version of their site - and not test it with an actual phone. I've found that a lot fewer than 50% of "web developers actually know what they're doing. [I hired one once - who lasted 2 days - it took him that long to ask me for help for something that made me realize that picking someone out of a bar at 2 AM would probably have gotten me someone with better programming skills - even when he was drunk.])