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Hi @skittykat I understand how frustrating this can be, I am not aware of any specific issue between certain networks, we will need to take a look at your account to see if there has been any progress from the tech team in regards to your case, as we can`t access account details on this forum if you can contact us here and one of the team will be able to pick this up.
It sounds as though you may have the Chat option set in the messaging app, which sends messages over data rather than the SMS system. I've tried this and found it to be slow, so turned Chat off. It means you can only send plain text (unless you use MMS), but as there are other options for enchanced content (email, social media etc), it's not a major problem.