The "otherwise unused" isn't true. Any time your phone is transmitting, it is using some of available bandwidth in the air. It does go other the control channel

The "otherwise unused" isn't true. Any time your phone is transmitting, it is using some of available bandwidth in the air. It does go other the control channel is GSM rather than setting up a data channel, and is was originally just for testing, carrier announcements etc. The per-SMS charge is far higher than actual costs, but so is everything. Mobile networks are almost entirely fixed costs in building the network.