Been with TalkTalk for a few years and after the leak of people personal details a few years ago i've had to put up with scammers saying their from TalkTalk technical support sometimes 2 or 3 times a day so the other day i decided to leave and move to a new supplier and guess what i got a scammer phone me today saying their from my new supplier, the only people who have access to that info are TalkTalk and the new supplier "very strange"
I'm very interested in what you've discovered. Another member recently related that a visiting Openreach engineer believed that a scam call incoming to the customer when the Openreach engineer was there on a repair call was because someone was revealing customer telephone numbers gained from Openreach.
In your example, if you're switching to another ISP using the Openreach network then Openreach will have been given your number.
As ferguson says, report this to your new ISP so that there's no delay in investigating.