Most towns are good.  Skagway can be very spotty.  I usually go to the highest deck of the ship, and it works.  Make sure data roaming is turned off and you won't pick up the ship's cell service.  There are numerous places you can get cell service while underway.  There's a cell tower in Gustavus near Glacier Bay and as you go past Hoonah.  AT&T has the most towers and best service.