on directivos, guide data comes down via the satellite. the phone home via phone or ethernet is just to get software upgrades and send your usage logs to tivo. there is also the seperate phone call which goes out to dtv to let them know about your ppv purchases.
fixsub will cause the tivo to think it has done its phone home. if you don't plug in the phone line, then you won't be able to order ppvs from your remote (you can order them online though).
in general, its a bad idea to let your tivo call home if you have bash and such enabled. the reason is that you are subject to being upgraded, which is not something you usually want when you have hacked your unit.