Comment by toast0

Aggregators do some of this, and they can negotiate pricing to some degree, but a carrier is unlikely to intentionally give them zero cost traffic, and even if they do, they're not going to pass that through at zero cost.

I ran the engineering side of carrier integrations at WhatsApp. Carriers wanted to sell data plans with special pricing for data with WA and use WA branding in advertising, because it attracted customers that might later convert to a bigger general purpose data plan. As part of that, we would ask for zero rated SMS to their customers for verification. When it was available, it was generally faster and higher success vs sending messages through an aggregator.

We also had some, usually small, carriers approach us asking us to set up direct routes to them for verification, because their customers would not always receive our messages when we sent through an aggregator. Early in my career at WA, we would just send these carriers to our aggregator contacts, and often things would get linked up and then we'd still pay $/message but it would work better. As we got a little bigger and built support for direct routes anyway, it was usually not too hard to set up a direct connection and then there'd be no cost for that carrier. Messing around with IPSEC VPNs and SMPP isn't fun and the GSMA SOAP messaging APIs are way worse, but once you get the first couple implementations done, it becomes cookie cutter (and FB had built way better tools for this, and a 24/7 support team, so I never had to be up, on the phone with telco peeps at 3 am kicking racoon or whatever ipsec daemon we were running until it finally connected)