I have been having the same issue on all calls, not just on speakerphone, on my brand new iPhone 13 Pro since I purchased it in September. I've had multiple chats with the Care team at my wireless provider, Visible, and also with Apple Care. Finally went to an Apple store and they replaced the body of my phone (using the original screen from the phone I purchased, which is apparently something they do to save on screen replacement costs), and the problem continues. During voice calls, the people I talk to say that my voice breaks up and they can only hear every few syllables). Super frustrating. Back to Apple Care for more troubleshooting. I suspect it's a radio problem in this model, but have yet to hear Apple say as much.