I have a paid Twilio support account, but they have been awful with their response times. I'm at my breaking point with Twilio. Are there any alternatives that you've used successfully?
If you have a feedback loop to indicate successful receipt of messages, the best thing to do is to use multiple providers and track success by destination carrier and route to whatever provider is working best at the time. If you run graphs on this, you'll find out lots of interesting things --- like when one aggregator goes down, you can see which other aggregators are using them.
I don't want to endorse any providers, but if you're looking for messages to the US, the list of partners from the shortcode registry is a decent place to start https://www.usshortcodes.com/partners/find-partner
Bandwidth is also one of the underlying provider for a HUGE number of telecom companies. We switched from Twilio and haven’t looked back.