The US is pretty weird for mobile stuff. Here in Europe, as far as I know, you basically just pop a different SIM in a phone and everything pretty much works. Also, you don't pay when someone calls you.
By the way, that's why it was easy for Twitter to provide free sms forwarding in US (first). Operators would easily agree - they'll get the money from recipient.
The US problem is stupid carrier exclusivity!
Incredible how country that values freedom so much allows such lock-ins to exist!