This is actually pretty common in hypercompetitive commodity markets. The largest player has a slight cost advantage due to economies of scale, so they have the best price and everyone buys from them. But they still have no market power because their market share doesn't come from barriers to entry.
> Err, yes.
It's not necessarily a monopoly even at 100% when there are competitors who could immediately enter the market if the incumbent were to be so audacious as to raise prices by 4%, or do anything else the customer even mildly dislikes -- because that fact keeps them from ever doing it.
Notice that this is not how it works for Comcast, because it's not cheap or quick to wire a city with fiber, so they can get away with a great deal of abuse before anyone else would show up to compete -- even if they only had 50% market share, as long as the other 50% is another company doing all the same abusive stuff.
> And AT&T wasn't a monopoly, since you could just walk to the person you want to talk to. Oh, wait..
To be a substitute it has to be a practical alternative that can be used for the same purpose at approximately the same cost. Having to spend an hour walking is not the same cost as picking up the phone.
