Isn't the hard prt the connectivity bit i.e. negotiating with the various telcos? I once saw a telco use a third party SMS vendor for messaging their own customers for an app - because setting it up internally was too much of a hassle.
We connected to multiple aggregators. It's been a few years, but the big players in the US (Verizon, AT&T, Sprint, T-Mobile) were split between different aggregators. It was a similar situation in Europe.
A big part of working with a new aggregator was a full review of security and privacy, and that became even more important as we began the process of being acquired by an F100 company.
I'm still trying to figure out why messages were stored in S3 buckets to begin with. That's an architecture choice that makes little sense to me, especially since the limited size of SMS makes them pretty space efficient.