>> All traffic passing through Twilio is categorized as application-to-person traffic.

Thanks, I didn't know that.

>> If you haven't registered your brand, been assigned a trust score, and registered your specific number with a specific a2p campaign, you'll be subjected to heavy filtering.

I've done none of the above. I've been using Twilio for 3 years and this is the first time I've ever had an SMS rejected. I don't get it.

>> I think voip.ms only allows person-to-person traffic. Try registering a number there and sending the same message that got blocked.

I'll give that a try.