I'm considering in a similar switch (from BT -> PlusNet). The PlusNet switch page should help
You will need a MAC,
Remember that asking for a MAC is not equivalent to telling the from company you are cancelling.
When you have decided to switch and got things ready with PlusNet, inform TalkTalk to make sure they are aware.
I'm not sure what the situation is that might cause loss of landline number, I'd be pleased if anyone could clarify that (I don't want to lose the number we have had for 35 years).
BT Infinity 2, thinking of moving to PlusNet