[TimBurman]

You could try logging into Gmail just to the point before the SMS is sent, then phone the number you used to have and speak to the current owner. Apologize that you forgot to take the number off your account and so they are receiving text messages meant for you, but you would like to get one more message from them and they won't be bothered anymore. Then you will switch to a new number on your Gmail and they will not receive your messages. Many people will be happy to help a stranger if you are nice and polite to them.

The other option is to contact the phone company and explain, asking an open ended question if there is any way they could help you, with the permission of the current owner of the number, to get one more text message and move your Gmail to a new number. It doesn't sound in any way like you are trying to pull a scam, so they might help.

[qingcharles]

I've tried for 5 years now on-and-off to talk to the current owner. They read my texts but won't answer them and don't seem to answer any call from a number they don't recognise.

I also tried to get blackhat and tried to find anyone who can intercept the text, but I've not been able to, even on some rather shady forums...

[TimBurman]

In the United States, AT&T Wireless has an Office of the President, while Verizon has Verizon Executive Relations. Other companies might have something similar. I remember the Office of the President doing some amazing things for customers, mostly related to billing that were the customer's own fault, when nobody at any tier of the call center could help them. Even calling your own phone company, they may have internal contacts at the old number company if it is different. You could also try the executives at Google or their legal team, who almost always respond to certain types of requests. When I worked for government, other governments would bend over backwards to help us, maybe companies do the same for each other or would respond to someone from the FCC or another level of government. Just brain storming but you seem like the kind of person that will eventually figure this out.