What type of jack is the flight interphone on transport category aircraft?
The jack on every commercial airliner from Boeing and Airbus is a single plug U/174 type (aka helicopter). 

That holds true as long as we speak of the external jacks used for the flight interphone and service interphone. In the cockpit this is a different beast. You'll find a lot of variation:


classic two-plug 
U/174 (aka helicopter, impedance must be matched, no power to headset)
XLR-5 (passes power to headset, often found in Boeing aircraft)
NeutriCON (passes power to headset, often found in Airbus aircraft)
LEMO (passes power to headset, used mainly by Bose)


What you'll find in the cockpit mainly depends on what the buyer ordered. Boeing standard in their newer aircraft is the classic two-plug and a XLR-5 side by side (e.g. B747-8, B787, not sure about the B777). As an example, the company AvtechTyee supplies their 5775 series jack panel for the B787 First Officers side.