Can iPhone 12 be used safely in pouring rain?
According to the Technical Specifications of the iPhone 12, the splash, water, and dust resistance is rated as follows:

Rated IP68 (maximum depth of 6 meters up to 30 minutes) under IEC standard 60529

(I picked the minimum ratings - iPhone 12 as a baseline)
What does this mean?
IP68 ratings means the following:

The first digit, 6:  Protection from contact with harmful dust
The second digit, 8: Protected from immersion in water with a depth of more than 1 meter (manufacturer must specify exact depth)

For dirt and debris, this means that some dust that's kicked up from a gust of wind is fine.  Going through a sand storm in the desert is not.  As for water, this means you can safely submerge your phone in water about 18 feet deep (deep end of a pool) for about 30 mins (10 mins longer than the 20 required for you to stay out after you eat).
Can you go swimming with it? Not really. These ratings are for water and it doesn't cover chemicals like chlorine or the salinity of ocean water. Can you drop it in your tankard of ale? Nope. It's not certified for booze.
Can you use it in the shower? It depends, if it's just the standard (builder) shower head that you get &quot;with the house&quot; you're probably fine.  If it's a fancy massaging shower head that pressurizes the water with those oh, so pleasing pulsing jets of water, then no.
As for a rain storm, if you get wet while in a light to moderate shower, you should be fine. But if you're thinking of continuing your conversation in an all day downpour or while you report on a hurricane/tropical storm making landfall, then no, it won't survive. Remember, even submerged, there's a maximum amount of time it can stay submerged.
TL;DR
It's water resistant not water proof. Apple's technical specifications tells us exactly what the resistance levels to dust and water are. If you happen to get wet (in the rain) while using your phone, you should be fine. They tell you how to dry it off and how long to do so before charging because water conducts electricity. This is to prevent short circuits when connecting up to 20V to your phone.
Besides, talking in a heavy downpour is logistically next to impossible. It's loud - the person on the other end will have difficulty hearing you as you will with your caller. As a resident of Florida, I can attest that a severe downpour while sheltered is still very difficult to hold a conversation.
If you're using the phone in the shower....that's a whole different conversation for a different Stack Exchange site.