asterisk users - Mar 2010 - No audio when calling via PSTN, before remote answers (with polarity reversal)

Hi!

I want to get audio from the PSTN before the call is answered so I don't
miss
when the called phone is busy or if there is some error (like the phone is
unavailable or is wrong, etc) and hear the ringing from my telco.

I have polarity reversal in my telco for incoming and outgoing calls. 

If I set answeronpolarityswitch=yes then I get no audio until the call is
answered. If I set it to "no" it works fine sometimes, but other times
when
the call gets answered, asterisk detects the polarity reversal as a 
hangup and hangs up the call.

I need to have hanguponpolarityswitch set to yes for detecting hang ups in
incoming calls. (it was a nightmare before this)

Any ideas?

I had this working in previous versions of asterisk but didn't find what
was the change that caused this behaviour, and I need to use a recent version
of asterisk for some changes in dtmf caller id detection that aren't in my
distro yet (debian).

Thanks in advance.