Three things
All of it, from your side
What actually happens on the day
The four things people ring up to ask, answered before you have to.
About your landline
The old phone network is being switched off, so every landline in the country is moving onto broadband. Yours will too, whoever you're with. It keeps your number and it works the same — but because it runs over broadband, it needs power. If anyone in the house relies on that phone, say so when you order and the battery backup is free.
If it doesn't go to plan
Switches do occasionally go sideways, usually because of something at the other end. If yours does, you ring the same number you'd ring for anything else and speak to somebody here who can see your line and chase it. You won't be handed between two companies each saying it's the other one.