On the old AMPS phones that YOU, not the company, controlled, you had the
choice of which carrier you put the phone on.  "Home only" meant the phone
would only connect to your home system, which in those days was just your
city as the plans only covered a small area so you could be charged
exhorbitant rates if you dared travel to anywhere.  To prevent the phone
from roaming, you left it in "Home Only".  The other two choices allowed
roaming and which one you chose depended on which of the two old cellular
bands your home carrier used.  If you had a carrier on the B band of
channels, you left it in "Automatic B".  This meant the phone would search
for the B carrier before searching for the A carrier so at home it would
find your home carrier....unless, of course, your home carrier had a hole
then it would roam over to his competitor and you would be charged roaming
rates in your own front yard.  It was your choice, not theirs, and it
depended on how much value you put on having a phone that worked.  When you
traveled, it would still be looking for the B carrier first, even if your
home company was on A band in the distant city.  It was to your advantage
to roam on your home company because his roaming rates in the distant city
were only $1.65/min, instead of the B carrier's $2.88/min.  So, everyone
soon figured out how to change the band priority or get screwed
worse....(c;

In digital cellphones, what the phone uses for roaming, or not roaming or
just saying NO SERVICE in the coverage holes at home is THEIR choice.  
That's what the PRL, Preferred Roaming List, instructs the phone to do.  
This was supposed to be a "feature" of digital, but is now a way to deny
you service on competitor's systems.  Alltel, in Charleston, SC, where I
live, will not roam to Verizon or Sprint PCS at all if you are in a dead
spot.  Verizon and Sprint are the same way.  This keeps the companies from
having to pay the competitors for service in those parts of the "rate map"
colored in, but is a lie on the "extended roaming free networks".  They all
switched from the free roaming after the reality of how poor their
individual services were covering where you drove around, roaming to the
other guys' systems and costing them revenues due to the holes.  So, they
all simply stopped the roaming with the PRL programming.

It sucks, doesn't it?