That looks right
Also note that I think it would be simplest to just solder a phone cord "pigtail" to the guino that is long enough to reach that jack installed under the dash somewhere, where it could still be unplugged if needed.
FYI, I'm considering moving to a pigtail arrangement just because A. these plugs are a pita to make by hand, and B. having a phone jack on the board will make the unit noticeably larger.