asterisk users - Aug 2007 - Weird noise problem on SIP transfers...

I'm wondering if anyone has seen (heard!) this before. I have a site which 
has Grandstream Budgetone 100 phones (don't laugh, they weren't my
choice
and I was quite angry when I heard they'd been installed )-: They have an 
asterisk box with a TDM400 card in it with 4 FXO ports and 4 lines to the 
telco (BT, in the UK)

It basically works and does what it says it's supposed to do on the tin. 
This company have 3 "offices" the PBX is in the main one and 2 others
have
ADSL VPNs with a total of 12 handsets.

So they can call each other internally and the sound quality is fine. An 
incoming call from the analogue lines sounds fine, as does an incoming 
call from an IAX trunk.

But sometimes, when they transfer a call, they get a lot of noice/hiss 
after the transfer. It's heard by both the new person and the original 
caller, and I can't work out where it's coming from. I have heard it 
myself when I dialled in on an IAX trunk and the first person I spoke with 
was clear as a G711 clear thing, but when it was transfered, both myself 
and the new person I was talking to heard the noise/hiss.

So I'm preplexed as to it's source! I don't think it's anything
to do with
the analogue side of things as it happened on a purely digital call, and 
I'm sure this is nothing to do with the VPNs either as I heard the noise 
when it was transferd to someone in the main office.

Anyone heard it before? Is it really the BT100's being useless? It
doesn't
happen every time which makes it even more perplexing. (I have lots of 
GXP2000's which work very well, but I've only ever played with a pair of
BT100's myself and don't recomend them, but this was installed by
someone
else, alas.)

Any clues would be helpful!

Thanks,

Gordon