I had the same issues. Ran a side by side analysis with Nexmo and Nexmo were winning. Contacted Twilio customer support with the data and they "changed some rou

I had the same issues. Ran a side by side analysis with Nexmo and Nexmo were winning. Contacted Twilio customer support with the data and they "changed some routes" and deliverability was much better, outperforming Nexmo by some margin. Lesson learned: Create a test case, then contact support with evidence. It's a lot more effort of course, but now I have a system which can flawlessly switch between services if one begins to fail, and Twilio were much more inclined to help!