> In short, reliable delivery of SMS messages internationally is still not solved.

So how does MessageBird solve this...?

see my comment above.

So that error doesn't exist in MessageBird? I don't get it. Is it not dependent on carriers as well? I'm asking how they technically do things differently that would absolve them from encountering that issue.

When you connect directly to a carrier you get so called PTT codes back (industry standard). Not all carriers do this the same way but we standardize this and return them like shown at the link below. When your provider uses a lot of in-between providers its harder to return errors correctly. https://developers.messagebird.com/docs/smserrorcodes

Do they use any tools to monitor sms reception on real SIM cards?

Yes - we run our own network of real sims for quality control that we only use for ourselves.