SMS is pretty much decentralized, although there's a few companies with a lot of reach. I don't remember any Global SMS outages, but it wasn't uncommon for a whole carrier to have an SMS outage and especially for inter-carrier SMS to be broken from time to time (sometimes for days). I've certainly seen some stuff with SMS aggregators: almost all of them claim a majority of direct links, but when you have accounts with 4 large aggregators and one of them has an outage, you find out which of your other account use that aggregator for which links (because their deliverability will go to zero to those destinations).

RCS was designed and specced, by GSMA, as a telco run decentralized system that would replace SMS as like for like; but there were only a handful of rollouts. It's really only gotten use as Google pushed it onto Android, using their RCS server; recently iOS started using it although I don't know what server they attach to.

Since RCS is basically the 5th wave Google IM, it's no surprise when they have a major outage, RCS is pretty much broken.

It used to be kind of distributed, but Google has been strong arming carriers to use their hosted Jibe service through a combination of proprietary extensions (e.g., E2E which is finally standard) and bypassing carrier control (if the carrier didn't provision RCS, Google Messages would use their own service iMessage-style).

From the end user's perspective, if the carrier didn't use Jibe RCS, it simply wouldn't work well.

People liked to be utterly pissed at Apple for not supporting RCS. But there were reasons