iPhone speakerphone doesn't work after update 15.3.1

I am also encountering the same issue. Yesterday I updated my software and next day the ear speaker is not working.

what is the fix ?

I also had the same issue on an XR after updating to 15.3.1.

I looked up the on-line advice and carefully cleaned out the speaker holes, ear speaker hole etc. I called Apple customer support to see what the advice was and they said it was most likely a hardware fault and the cost of mail in repair was £380 ($500)!

I booked a Genius Bar appointment at the nearest Apple store to double-check. The ran audio tests and determined that there was a problem with both the ear speaker and the mic, cleaned it properly and Hey Presto!, it now works fine again. Well worth the journey to fix this!

I have a theory that the update in someway changed the sensitivity of the software feedback from the mic/speaker which reduces the volume, hence why the "fault" appeared immediately after the software update.

To rule out a software issue, set your phone back to factory settings, using your computer.

Then set it up manually, without using the backup see if you can make phone calls as usual.

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

If it works again, restore from the latest backup to get all your data and settings back.

I had no issues after iOS 15.3.1, but I always use my computer to upgrade iOS, since upgrading over Wi-Fi only replaces the new parts of iOS, which might cause software issues in some cases.