Well there's the precis.
We're with BT. My understanding is that if we don't do anything we will keep our landline number. We can pay for internet and 'phone or internet only. There's a difference in price. It's up to us to work out how much use we do make of our landline and how much it will now cost. It could be cheaper to transfer (port) the number to a VOIP provider.
We would then change to internet only.
But long and short, for us, doing nothing means it will all still work.