Speaking of "carriers" in general, rather than just AT&T with the iPhone: IIRC, on Verizon my parents paid $30/month extra -- above and beyond the data plan --

Speaking of "carriers" in general, rather than just AT&T with the iPhone: IIRC, on Verizon my parents paid $30/month extra -- above and beyond the data plan -- for unlimited texting, because some of their business contacts are inveterate texters who won't or can't currently switch to email on their phones. They are already paying Verizon's top tier prices, including for unlimited data, and then get walloped an extra $360 a year for the privilege of cramming brief messages into the spare room of the network's status/heartbeat messaging. I mentioned a Voice account (or two), but that was a bit too far afield for them, at the time (and imagine their contacts' confusion over a second number). Nonetheless, saving $30/month seems like a bit of motivation, in general.