asterisk users - Dec 2005 - Echo on incoming sip provider- asterisk - sip phone / ata

Hi,

I am having intermittend problems with echo on the line (sometimes a 
clear connections, other times lots of echo on the line), especially on 
incoming connections from my sip provider.
Dialing out through this provider (budgetphone.nl), is most of the time 
no problem, dialing out through other providers (like voipbuster) is 
more stable.

On my client side I am using  Sipura 2002 and 3000 devices and  a 
Grandstream  2000.
The Asterisk is running on:
Asterisk 1.0.7-BRIstuffed-0.2.0-RC7k built by kk@nyx on a x86_64 running 
Linux

1.
Is it possible to improve sound quality by tweaking the Asterisk, or is 
this echo a problem between the Sip provider and the client devices on 
my side?
2.
How can I check (preferably in the logs) which codecs  were used for a 
specific session.
3.
Is it possible to force incoing connections form an specific  incoming 
connection  to us a specific codec? (I am currently using a list of 
preferred codecs ulaw, alaw and gsm.)
4.
What ata's are "better" in removing echo???

Any help apreciated.

Bernard.


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