Speaker grayed out

Make sure the phone is up to date firstly.

I've found with some people that a reset all settings has rectified this issue. Settings -> General -> Reset -> Reset All Settings.

This does not remove data but resets the settings on your device to factory. your photos and such will still be there.

If that doesn't work for you then a restore could be your only fix for this. make sure before you do anything like that to backup your data (preferably with iCloud just in case is the software on the phone having the issue.

I have tried options 'Reset All Settings' as well as 'Erase All Content and Settings', still unable to speak with anyone over phone. Speaker option is grayed. Issue started post OS update to 12.4.

I took phone to Apple Store after checking hardware and software, team had only option to get a new iphone7 in replacement which doesn't make any sense to me.

When your software corrupted my phone, why would I pay again for the same phone and brand. There should be a fix from Apple to this issue.