Rtptimeout parameter not having effect in 1.6 and 1.8

Hello all,
This is my first post, hope i am not stepping over any boundaries !

In a recent installation of Asterisk 1.6 and 1.8, I would notice some zombie channels from time to time
I believe such issue used to raise in old versions of astersik …
But the (rtptimeout) parameter should have taken care of it since it was introduced.
I used to rely on it and watch it work for me in 1.4

But now in 1.6 and 1.8, changing the value to anything is not affecting the behavior, asterisk keeps waiting for the BYE signal.

Had this feature been removed, or is there any other parameter that is affecting the behavior and i am not aware of it ?