After stripping the phone, and a few Google Searches, the culprit has been filth from the previous owner.
To rectify, a fill tear-down was undertaken in which I was able to remove the filth from the earpiece speaker and from the grille internal.
A pin was then used to clean the microphone hole at the bottom of the phone. Should a pin not be readily available, a staple is thin enough to clean the hole too.
Once cleared, this resolved my issues with the phone.