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 Pickup - Imelda / '08 Tacoma TRD - Donna
'09 Kawasaki KLR650
'98 Rhygin Juke SS / '12 Gunnar Rockhound 29 / '16 REEB Dikyelous
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein