This would be because the AVM Fritzbox is broken.  Its not G722/16000 its G722/8000 (even tho its a 16k codec).

On Jan 20, 2012, at 10:28 AM, Roland Hänel wrote:

It seems that FreeSwitch cannot negotiate G.722 with an AVM Fritzbox device
(*the* most popular VoIP box/router in Germany) because of what seems to be
a concatenation of several bugs.

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Brian West 
FreeSWITCH Solutions, LLC
Phone: +1 (918) 420-9266 
Fax:   +1 (918) 420-9267