Use an Analog Telephone Adapter to power your existing phone jacks in your home or business to use VoIP service in every phone jack.
Make sure that you do not currently have phone service since it may break your ATA while voiding its warranty
Reuse your existing wiring and telephones
Depending on your ATA, it might not be powerful enough to drive all of your phones
Some phone companies unplug the phone jacks from the point of demarcation when you close your account them them so that your jacks are no longer connected together and would need to be rewired (which is illegal in Canada).
If you are in Canada, we cannot legally advise you to do this.