We’ve had some intermittent problems with Asterisk shutting down at our K2ETS remote site, and I’ve discovered something important. The Raspberry Pi MINUMUM operating voltage is 4.8v. The USB cable you use to power it is CRITICAL. To check,  I used a USB cable which I cut and stripped, and measured across the red and black conductors. I plugged this into a USB port on the Pi. I checked 4 cables on a Pi running with one URI connected, and got 4 DIFFERENT Pi voltages. The lowest reading was 4.5 volts on 2 cables! One read 4.7 volts. Finally, the fourth one read 4.92 volts. These cables are not rated for current, so you must check them!