If you are referring to senderID registration (or 10DLC in the US), then things are getting really complicated thanks to the increase in smishing and the government regulations trying to stop it. In many countries, there is a significant amount of paperwork - and sometime fees - to do to allow your traffic to flow. Along with this, SMS margins have been really tight in the last 4-5 years (think 2%-3%), so these companies need to charge you for this paperwork and many times the rejections are not coming from them, but from the telco regulatory authority in a given country.
source: running a SMS service for the last 20 years.
I don't mind paying a premium. I'd happily pay 3x Twilio's messaging fee if I had no BS to go through.
In my experience, Sinch has the best registration workflow (everything online, document templates and knowledgeable support team). Vonage is next.
What is your main use case?
Do you have a marketing use case ? If so I would be eager to understand your main pain points.
Nowadays, it's expensive and contains a ton of hoops to jump through for business and a2p verification. They charge you for many things (monthly for a number, for verification, every submission for a2p and re-submission if they reject it for whatever reason). I tried to contact support and they wanted me to upgrade to a support plan to even ask a question.
If I don't find a simpler solution I'll remove transactional SMS messages entirely from my application. I'd rather remove it than deal with Twilio.