June 2010 - latest information on this device
There is a known problem with the ZyXEL P2000W and P2000W_V2 SIP Phones.
In SIP messaging, the ZyXEL phone is passing a parameter called a "cnonce" in a fashion which violates the SIP protocol specifications.
Upgrading the P2000W phone to the latest version of the firmware (as of June 2007 - WJ.00.13 and can be found here) seems to resolve this issue. There is no known firmware patch for the P2000_V2 variant.
In addition, by default this phone defaults to G.729 protocol. It is strongly recommended that you manually select G.711 or GSM encoding. OnSIP does not support G.729. Instructions for changing the default encoding can be found in section 13.5 of the user's manual.
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