I've had VOIP and an old all-in-one for years. Look at your cable modem. If iyou can connect your fax/all-in-one to the cable modem using a standard phone jack , then you probably can fax using VOIP. You don't even need a PC turned on to use your all-in-one for faxing/photocopying.
If your cable modem does not have a provision for a phone jack, maybe you should inquire from your cable company if they have one that does. Ours was pretty much plug-n-play.
Basically, the idea is to try to connect your old phone line/s to the cable modem, rather than the old land line box. Only now, you're using VOIP instead of POTS.
some old Tips: http://chris.pirillo.com/how-to-fax-...-the-internet/