My ring tone always rings loud first ring, then digresses to silent

Ever since I got this Iphone 11, the ring tone changes from loud to lower after first ring. I've contacted Apple and they can't figure it out, walked me through all the settings to make volume maxed out. Still when a call comes in, it may be very loud first ring and then goes lower ... sometimes it's very loud and stays loud... there's no rhyme or reason to this ring tone.. Have chosen different ring tones, still does the volume thing up and down.. even when checking volume is at max.