You connect a voip box to the modem or router..
I can plug any phone I want into my voip box, and currently have a BT phone with answerphone plugged in.
I have switched off the voip answer service.
Before this I had a cheap and nasty £3.00 asda phone plugged in and the voip answer service on..
Choice is yours, and depends on who your voip provider is.
It is a minefield out there, which is why I went for vonage. they send you a box. you plug it in. you put any phone you want into the box.