For those people who get dropped calls with the Samsung Galaxy S4, try this: SETTINGS, MORE NETWORKS, MOBILE NETWORKS, NETWORK MODES. (I think that's the last one) Switch from the top 4GLTE to the 2nd one. I went to the T-Mobile store and they said T-Mobile is just rolling out the 4GLTE and it isn't up 100% yet. Give it another few weeks and it should be better. I don't know about other carriers, but try this and see if it helps. If you can only talk 2 minutes, it certainly can't hurt right?
I should add that the T-Mobile store is probably 2 miles from my house and she showed me with one of the display phones. It was barely at one bar and went to full strength when she did it. I texted my Mom and Stepdad and he texted back saying he had 4 bars and no lower than 3 bars at any time.