Grass isn't always greener with the prices from other ISPs.
My dad is the account holder and he uses the landine a lot and pays for the according addon for the landline.
If he went to BT for a similar landline package he would end up paying more for it from BT and as for the broadband speeds, the BT/Openreach network doesn't come close to the speed that he pays for from VM and my area is FTTC from BT/Openreach.
Such is the lottery of trying to renegotiate with VM, which has become like the back-and-forward sales process of buying a used car from a dodgy car dealer.
Some people do well when they call in and get what they want. Others hit a brick wall of non-negotiation (depending on whatever VM's targets and offers happen to be on the exact day/time the customer makes a call). Also depends on the customer's alternative options.
I have just moved from BB+phone with VM (who would not offer BB only at an acceptable price and were insistent on pushing BB+phone) and switched to BT FTTP for BB only, increased download and upload speeds and more than halved monthly costs.