Mobile phones aren’t perfect, no matter which network you’re on. There are dropouts.
What’s even more annoying than a dropout is when both parties try to re-establish the call simultaneously, and so both get busy signals or voicemail. And they keep trying.
In my book, unless someone’s said beforehand that they’re the one who will ring back (as in “I’m going into the tunnel now, I’ll ring you back”), it should be the person who originated the call who does the calling back.
If everybody followed that rule, it would prevent this telephonic Mexican standoff.