That's certainly what the proponents of flat design contend. In terms of measuring usability, I'm not so sure that's true. I've seen people hunting for buttons that didn't quite look like buttons
This is the beauty of the traditional, three-dimensional, consistently-styled buttons: you get a 'button' by default without having to hint that it's a button using other means.
And the Call-button is for whatever reason merged with the SMS-button. It's just one big button of which 4/5 is for calling and 1/5 to the right opens up the SMS-app. There is an icon representing SMS in that 1/5 of the button, but nothing indicates that it's a separate button from the 4/5 on the left.
iOS Contacts has a set of icons that look like the symbols on the app icons. That makes sense to me.
The classic example of good interface design, I was taught, was the "call lift or elevator" button. The theory goes that no one needs to be shown how to use a lift twice. You see it once and just know what every other lift button does even if they look slightly different.
I realise that that skeuomorphism needn't necessarily map to the computer world but _something_ seems to be forgotten in the current trend for flat, non-skeumorphic designs.
I loved hearing that Windows 10 was going to go 'authentically digital', but that emotion died when it became apparent that they decided that mean 'flat design'. Just because something is on a computer screen doesn't mean it has to be made of simple single-colour polygons.