Hi @Kez82, sorry to hear about your phone. 
Based on the information you have provided it does sound like it's the main microphone that is the cause of this, since when you put the phone to loudspeaker the second microphone is being used instead of the main microphone.
I'd recommend that you start with testing both the microphone, ear speaker and the loudspeaker in your phone. You can do so in the inbuilt test in the phone called Diagnostics which you can find in Settings > About phone > Diagnostics > Test.
If they work fine in the test, try rebooting your phone in safe mode and make a few calls. That way we can see if this is caused by a third party application, since these applications won't be loaded in safe mode.
If there is no change in normal mode or safe mode I'd recommend that you try and change the network type in your phone and see if this only happens when you make calls with 4G/3G.
To change the network type, go to Settings > More > Mobile networks > Preferred network type > GSM Only > make a few calls.
Let me know how it went!
Thanks for the update!
Sorry to hear that.
If you have another SIM card at hand or if you can borrow one from a colleague, friend or family member that would be great, so we can rule out that this is caused by the network.
If there is no change with another SIM card, I'd recommend that you perform a software repair using Xperia Companion. This will delete all data stored on the internal storage of your phone and reinstall the latest available software.
Since data is being deleted, creating a backup using Xperia Companion before starting with the repair process is advised.
If there is no change with a different / new SIM card and after performing the software repair your phone needs to be examined at an authorized service centre.
I'd recommend that you get in touch with your Local support team and they will arrange a repair for you and answer any questions that you may have regarding local service routines, warranty and so forth.
Keep me updated!
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