>>>you want the same line to carry the same traffic, plus internet traffic, plus ip headers, plus voip/tcp/udp/whateverp headers. And you think you'll get something decent? Good luck with that.
Yes. The analog phone line is limited from 0 to 4000 hertz bandwidth. It's worse quality than AM radio (~10,000 hertz). If you do VOIP over a dialup modem, you can use digital compression equal to 48k AAC+SBR and achieve FM quality (0-to-15,000 hertz). So yes it's more efficient and it also sounds better.
Yes 48k AAC+SBR can sound as good as FM radio - http://yp.shoutcast.com/sbin/tunein-station.pls?id=520194