asterisk users - Jul 2006 - Asterisk and Vigortalk problem

Hi All on the list,

I'm having a problem with some Draytek Vigortalk, firmware v2.5.8 registered
to an Asterisk v1.2.9.1 (AAHv2.8 updated). Basically if I make a call from one
of those
Vigortalk extension to another whatever extension I don't hear anything
while the
called extension may do. If I reboot the Vigortalk I could make 1 to 2-5 calls
before
to get the same problem. The Vigortalk is working on RTP ports from 10000-20000 
as the asterisk is configured, also the other extension has the same
configuration.
The admitted codecs are ulaw, alaw for all the extensions and all the extension
are in
the same subnet.

So, does anyone is having/had the same problem and/or could point me where
I could fix such problem?

Thanks in advance,

Roberto Fichera.