The best idea I can muster is to use this https://www.sparkfun.com/products/14475 on this mic line, with the MONO enabled (input would be the normal mic wire wired to one of the input channels, and then output going to the wall…so, basically, you just wire this in the middle of the mic line)
Note the voltage range, though, not sure what the specs of the mic line is…this guide https://learn.sparkfun.com/tutorials/no … 1643964608 should help with any specs for the amp.
It might be easier/simpler to look for a phone that is known to have a loud mic