I don't SMS. Sorry. I don't even have a texting plan at all because I've never used it, never had a reason to use it, and all the texts I've received over the years were all spam. Maybe only a couple were legitmate, one when I was keeping a number alive via Google Voice, and another when Google or someone texted me a confirmation code (I think it may have been my carrier to confirm a purchase).
Now, I too only answer the phone when I recognize the number. However, I admit, I have a landline as well and expect people to call that and leave a voicemail (did I mention I don't have voicemail on my phone, either?).
And yes, I've also been caught by my own filter - I did happen to forget my phone one day and had to use a payphone. I left a message.
I never have to pay for incoming calls (unless I am roaming in another country) here in Europe. So there is no cost. Yet I have NEVER received a cold call on my phone.
Not once in the probably 10 years I have the number.
That's because in Europe, the caller pays, and to help differentiate the call rates, cellphones have a different prefix so you can tell when you're going to pay.
So of course people won't robocall a cellphone in Europe - why would you when it'll cost you 10 cents to make the call? Calling a landline is free, calling a cellphone is not. Naturally forcing people to pay will get them to not pay in the end.
Of course, in North America that's not feasible since a phone number can be a landline or a cellphone and there's no way to tell just by looking. Especially since numbers can go between the two for number portability.
Though, the carriers can implement caller pays by simply stating the called number is a cellphone and do they want to pay for the call.