SMS was originally piggybacking off unused bytes in packets already being sent to the tower, which was being paid for by existing phone bills. The only significant expenses involved transiting between networks. That was a separate surcharge in the early days.

Used to be? What changed?

People started sending a lot more texts and making a lot fewer phone calls. And you can only piggyback so many text messages on the call packets.

We don't use 2g GSM anymore.

> SMS was originally piggybacking off unused bytes in packets already being sent to the tower

for GSM? not really, Yes SMS was sent at a lower priority than voice, so if there is a lot of voice traffic, SMSs wouldn't be delivered as quickly.

But, to my original point, do you think the towers were free?

Thats the point, it took a shittone of captial to make the network.

Did the mobile networks make a lot of cash? also yes. But they also took on huge amounts of debt and went bust.