Use speakerphone / hands-free mode. Distance is your friend. Switch the phone to use the 3G network instead of 4G LTE. Usually phones need less transmit power when connected through 3G, but this may depend on the phone model. You can download a SAR measuring app and test it.

Mobile data will produce most of the radiation. Turn of mobile data, means your phone is not connected to the internet, but you can still make calls. This reduces emitted radiation significantly in stand-by mode (e.g. if you carry the phone in your pocket, turn off mobile data, and people can still text and call you, but your nuts are not getting irradiated by the constant internet communication).

If you're into new agey stuff, people think black tourmaline helps deflect emf.