What technical limitation?

There is nothing simpler, except that some very well-funded special interest groups never want to see this happen.

When I was on a limited plan, the amount of spam messages and calls were virtually zero because the limit would run up and be charged against the caller (internationally).

The month I switched to an unlimited, I started getting spammed.

Now much of the spam is domestic, but I fail to see how this is possibly unfeasible technically.