While I appreciate all the suggestions to change phone settings, it's clear that this is a Public Mobile problem. It shouldn't be dropping calls! A statement from PM that they're looking into it would be a good start.
You aren't the only one. My wife and I are both dropping calls like crazy, even to each other (both on PM). Here's more complaints : https://www.reddit.com/r/PublicMobile/s/Gzds7S1Izu
