[asterisk-users] Asterisk CDR "**Unknow**" as channel name

[Ruddy G.]

Hi all
I have been looking at my asterisk CDR in the mysql database and
some channel names are set to "**Unknown**" string.
When I look at the code, everybody when calling ast_channel_alloc set a 
channel format
like SIP/%s or Zap/%s
Only app_voicemail.c doesn't when sending emails and I don't use voicemail.
Why app_voicemail needs to allocate a channel to send emails ?
And in which case I have "**Unknown**" in the CDR
Thanks