Solutions to this iphone muffling and cut outs when you use your speakerphone have shown several remedies that fixed the problem for some, but not for others. These solutions have fixed this problem so far:
Update the iOS software on your phone to 15.2.1- Look in settings, general, software update.
Replace the SIM with the eSim ( I don't know how to do this)
Remove your case. Some cases block the microphone and speaker holes, especially if you are using a "waterproof" case. (This was my remedy and at least one other person on this thread.)
If there are any other solutions that have worked please reply. And, could somebody explain how to change the SIM to eSIM. Thanks and good luck all.
Also, there are people on this thread that have tried all these remedies and gone to Apple Genious Bars and/or talked to support and there problem was not solved so there may be some defective phones out there but there must be a lot of them and Apple hasn't acknowledged the problem exists yet.