Speaking of iPhones, I believe I figured out why the battery drains once or twice a month. If you guessed...
That would be wrong! I'm pretty sure it is downloading the newer version of iOS. Since I'm running an older iOS version by a few years, I believe it is downloading the whole iOS once or twice a month. I have data over cellular off. And now I have turned off WiFi. No more battery drains since.
What about my VTech phone battery drain? Odd coincidence. Weak battery. Purchased a new battery and it has been fine ever since.