Tried all the suggested methods to fix group messaging. They are sending but not as iMessage.
Also they are going to different Group messages or as single texts to certain people in the group. Not in the main group text.
iPhone XR, iOS 13
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Was told by Apple support that the phone number has been blacklisted because it was blocked many times.
The phone had been hacked and the hacker sent a ton of messages to random people.
Those people must have blocked the number, hence the blacklisting.
If it does not fix itself in 7-10 days we would need to change the number to not have the same issue.