Forget about dialing into a conference bridge, the other people on the conference call soon ask you to drop off because of the static you bring in. The only difference I can see between those choices is that analog can be generally annoying and digital bypasses that by frequently failing altogether. Even if the voice quality was generally good. The system can be hard to use. Roaming requires that you constantly tell the world when you are out of area and when you move from area to area.
Every system has a little different way of accepting that input.
Worse, my provider doesn't do a good job of maintaining system parameters as you move from area to area. Sometimes, the system stops rolling over into voicemail at all, which is really frustrating. I also had a little problem when I forgot to pay my bill and they cut off my service. But I suppose I can't really blame them for that. A universal voice messaging system.