Re: why would they need to steal the number?
"But they might want to transfer you to a really really really expensive tariff at their custom MNVO"
Not going to happen. This type of fraud doesn't exist as it would be simple to detect, simple to shut down and simple to send the directors to prison. Way, way to complicated to set up for very limited potential gains.
The energy transfers issue is easier to do and it is the 'sales people' doing it to get commission.
Once again, in the route of GDPR you have a nice new avenue with massive fines that you can also pursue to deal with these issues.