asterisk users - Feb 2010 - mysterious rippled sound with IAX

Hi all,

I know this could sound like a ghost story but...
I have prepared an Asterisk 1.4.26.2 PBX on a Debian Lenny, the same I 
always prepare, same hw, same sw. I connected it to 2 iax phones using a 
hub: it works!
I take everything it to our customer place, same PBX, same phones, same 
hub, same cables: the audio is rippled!
Changing phones, cables and hub gives the same result.
It does not happen with SIP phones but unfortunately I cannot use SIP 
protocol. :(
Tried with a trixbox 2.6.2.2 (Asterisk 1.4.22) on a normal laptop and it 
works.

It's like there something in our customer environment which is ruining 
IAX sound, maybe interfering with the PBX hardware (elettromagnetic 
waves?!?).
I know it seems illogical or not scientific but this is what happens.

Has anybody ever encountered a bizarre problem like this?

Thank you.

Giorgio