I find it frustrating when someone comes up with an idea like you have, and if you were to ask the ICO or any of the other regulatory bodies whether you were allowed to do this, they will not tell you and they will simply tell you to take legal advice. They do that even though they are the ones that would prosecute you for non-compliance!
I can only give you an opinion because there is no definitive answer to this.
I disagree that you not trying to sell them anything different. It must be different otherwise they would already have it. You are trying to sell your service.
You have already established that you will only be texting those who have opted in to the TPS so that is not a problem.
Provided in your message you quite clearly state that if they ring the number you have sent, they will get charged an amount specified, and which is over and above their normal call charge, I cannot see any problems for you. I can’t guarantee that no one’s going to take exception from it but I think you have covered all bases.
Can I clarify anything else for you?
It’s an important part of the process by which experts get paid.
We can still exchange emails.