For the time being the the list of Messaging providers is limited because the onboarding process is very manual and we do not want to have to hard-code many providers.
Now that we know that Messaging, the way we have implemented, works and is used, we plan on taking steps to refine the onboarding process.
We will probably though never reach the number of SIP Trunk providers we have for SMS/MMS providers, but we will add more in the future.
There is no ETA at the moment.