[Asterisk-Users] choppy and cracking sound from zyxel prestige 2002

[listasterisk.20.eenewz at spamgourmet.com]

Hi,

Does anyone have suggestions hooking Zyxel Prestige 2002 to Asterisk?

I have tested Zyxel Prestige with both supported codecs.

Call with G.711 sounds very choppy and cracking. Almost can't understand 
a word.

Today I installed G.729 support into Asterisk but unbearable voice 
quality remains. It's a little bit better though.

I have tested that Zyxel ATA with some commercial SIP providers 
(stanaphone, simpletelecom etc.) and the quality is OK.
tcpdumping connection with both providers showed that G.711 alaw/ulaw 
was in use.

Some Grandstream phones are also hooked into that same Asterisk pbx and 
using G.711. Those devices voice quality is superb.

Firmware installed in Zyxel Prestige 2002 is V3.60(MD.2)C0 (latest).

Please help.

Thanks in advance.