Using an older computer to provide private dialup = possible or not

RedPenguin

Yes. You need to set up the machine at home as PPP dial-in server. Another poster already mentioned the howto for that. You'll have to enable IP forwarding and possibly firewalling (iptables) on the machine at home. It's no different from setting up any other sort of firewall-routing-connection sharing. The IP masquerade howto at tldp.org is a good starting point. You'll just be routing between eth0 and ppp0 instead of two eth* interfaces.

Note that you'll only get 33.6 Kbps dial-up speeds. You can't do 56 Kbps dial-up without sophisticated equipment.

snip

Yes. The program listening for incoming calls should be able to detect the difference between voice and fax calls. Either 'vgetty' or 'mgetty' should be able to handle incoming voice, data, and fax calls handing each of the three types off to the appropriate software.

It's possible to have incoming calls sent to a software answering machine, certainly. But I don't know if it can be done in response to call waiting. It's been a while since I've worked with dial-up, but it certainly used to be the case that call waiting had to be disabled when using dial-up since the call waiting tone would be heard as excessive noise by the modem and cause it to hang up. Things may have changed since then, though.

There's some info on Linux software for answering voice calls at