The bearer path used an ip phone
to ip phones is rtp/udp/ip and hence digital
which mean no leak can occur.
However Acoustic echo is a major concern for
handsfree speakerphone terminals. The air
(and the terminal plastics) provides mechanical
or acoustical coupling between the
loudspeaker and the microphone. Speakerphone
manufacturers combat this with good acoustic design
of terminals, directional microphones, and acoustic
echo cancellers/suppressors in the terminal. However,
this is a very difficult problem, and speakerphones
are inherently poor in terms of echo.
The echo is probably introduce becaue you're using spearker phone
in a noisy area i.e a lab envirnoment.
try to call, using the handset and see what happens. Also if the phone
you're calling is not too far way from each other and one end is using speaker phone,
the echo would probably be a lot irritating ( louder).