Not with any general purpose audio tools, Avaya would have to provide something and they don't.
The C11 file isn't just an audio format, its an audio file (pedantic troll following who misses the point) with an additional header that contains information about the call or prompt (eg. mailbox owner, caller ID, prompt type, etc). So you actually need a tool that will remove that extra header info so that the file is recognised by other applications are being an ordinary wav.
...and of course you need to read all that header info before conversion, otherwise you'll end up with a pile of converted files but no indication of which file belongs to which mailbox user, etc.
Stuck in a never ending cycle of file copying.