Back to the original statement.............text doesn't use the same bandwidth as voice, and takes less of it, so it sometimes works when voice doesn't.
Something odd along those lines, my truck's On Star is Verizon, my in dash intellcast (text messages on the screen) is ATT. Why would GMC use both ???
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body. But rather, to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming...WOW, What a Ride!"