But what are the fixed costs of SMS? It's traveling over a network that was initially built to carry voice traffic, and is currently expanding to carry data traffic. How much of the network was built specifically for SMS? It really seems like it was something shoehorned into existing infrastructure. I have a hard time coming up with the fixed costs of SMS (at the carrier level; You could always add the cost of phone makers to build in support, but that's not the carrier's job).