Bombarded with spam SMS messages from the UK

I programmed a website to be able to send out SMS messages in bulk, for store owners, etc., who want to send specials to their customers (who signed up for it). The system worked great until not too many months ago many carriers, including T-Mobile stopped delivering most of these messages because of some changes in what is called A2P or "automated to person" type messages, and now we have to provide a whole signup process, sample text messages, etc., to Twilio in order to be validated to send A2P. If you don't do this, and if your text messages are flagged too many times as spam, you can have your number blocked, black listed, or disabled.

Here are my thoughts:

• These +44 (GB) country code numbers must be disposable, and they never receive your reply (message undeliverable). If the number is flagged for spam, so what? They get a new disposable number.
• The messages will contain another U.S. number that you’re supposed to reply to, or sometimes they tell you to use WhatsApp.
• What happens when you use WhatsApp or your normal SMS app to send a reply to the second number is that you have unwittingly and explicitly agreed to receive txt messages from that number because, after all, you sent them an SMS first. I think that's how they avoid accusations of spam: "He sent us an SMS first, so we did not spam him. He initiated the conversation, and we merely replied back."