Seriously though, they already give out coasters when you're in line waiting for a table. This would make it much easier to let the server know you need something. Half the time I'll be like "ask for napkins when server comes back." Then we forget when the server does come and don't realize until after they've left.
There is a local sushi restaurant in my city and they have little buzzers, it lights the table number on the display so any server around can see it and help you.
Several restaurant chains have little computer terminal things at each table where you can place an order and pay your check. Not sure if they have a call button or not. An even half decent server will check their tables often enough, or at least be in the dining room checking other tables often enough that you can grab them, so this isn't necessary. A shitty server that you might need this for will still leave you waiting anyway.
If the US is the only one with that on our food chains y'all are missing out