I'm not much of a focused writer, so other than comments on HN, and private communications. Not really.
If it's on topic, I'm happy to answer questions. And my email is in my profile.
On this topic, I guess the big learning if you use phone numbers for verification is to use multiple providers, and build a system that automatically chooses providers in real time based on success. They all have incidents, and they all have destinations that work better and worse. I don't usually mention suppliers by name unless it was clearly public, they put us in their SEC forms by name, so I think it's fair.
My "favorite" incidents are when one provider has a major incident, and several others have minor incidents that correspond exactly. This is because most of the aggregators are interconnected, and sometimes it makes more sense for them to send through another aggregator than directly (they don't tend to have direct routes for all carriers, regardless of what they claim).
If you use voice codes, I also recommend working to make the voice spacing sound good. Remembering 6 digits from a call isn't super easy; it's better if you make it sounds like two groups of three or maybe three groups of two. It's not totally obvious how to do that either; smashing audio files together doesn't sound great. You've got to adjust the silence between voice samples.