There's 2 mics. A lower one when on a regular phone call. And a Speaker mic when on speakerphone. They both require a new mic to be soldered on or by using a rework station to replace the old mic.
[link|https://www.ebay.com/itm/176629030848?|This is the mic you would need|new_window=true] for the VN220 whether it be the lower mic or speaker mic.
Though for the speaker mic, [link|https://www.ebay.com/itm/176368388358|you can replace the upper board|new_window=true] which has a new working mic on it, or of course solder off the old one and replace it with a new one.