You can use a Voice over IP analog adapter, but there are two important caveats:
- These are one-directional devices that assume you have existing VoIP infrastructure. They won't extend a normal analog line, but rather want to use an existing VoIP PBX.
- Fax signals often have trouble using these devices. Look for one that specifically supports faxing. Some of these devices are advertised to support fax machines, but as a practical matter will not, so do your homework.
If you don't have or can't setup a VoIP PBX, you can buy appliances that make it easy... but even simple ones usually run a few hundred dollars. You might find something like the Nettalk duo more to your liking.