I read this QA but it didn't specfically address my question. I also only found how to block a contact or someone that already sent you a message.
Say I have blocked numbers on my phone. They are not contacts. I don't want to hear from them. I don't want them to message me - ever.
How can I block someone when there's no chat between myself and them? (avoid potential harassment)
Or, alternatively, can I make it to only receive messages from people on my list?