Yeah, seems like a RJ11 switch box would deal with the mic. Don't forget about the speaker (the TNC drives audio out into the mic and accept audio in from the speaker). You'll have to unplug from the speaker jack when you want the internal speaker on voice or hook up an external speaker with a second switch.
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"To live is to suffer, to survive is to find some meaning in the suffering." -- Friedrich Nietzsche