Hi waqar,
To achieve the sort of thing that you get with
'conferencing' on a telephone, where you can switch
to 'speaker' and talk to the telephone like its
another person,
is actually quite difficult to achieve on a
do-it-yourself basis, there might be ready made
units you could buy.
however here is a simple arrangement that relies
on you keeping the gain low enough not to oscillate.
It is clear that the external mic will be able to
'hear' the ext speaker, but so long as the gain to
the earpiece is kept very low, it wont get back
via the mouthpiece to cause oscillation.
i havent drawn the press button wiring, i hope you
are ok with that.
It is not truly two wires, as it uses earth also,
but circuits for neutralising with only two wires
are not easy to set up.
Hope this helps.
John