I've always operated on the basis that the ptt operates your mike and cuts out all other conversations in the cockpit. Handy sometimes.
But as the man said it's all down to how the engineer decided to rig it. Be prepared for cross wired mikes. Also if you haven't heard the yakkity man on the radio for 20 or so seconds assume you are jammed on transmit. For that reason never refer to him as the 'yakkity man' whilst in the cockpit, or the bar....they don't like it....