Hang up immediately when dial "*0"

Hi all!
I have a problem when using channel dahdi.
First,I add an context “vmm” in extensions.conf like this
exten => *02,1,VoiceMailMain()
and I include it in the context of my dahdi extensions.
And then When I dialed *0 using a analog phone, it was hang up
immediately.It seems that the “*0” sequence be processed specially.
Is is a bug? Or how should I do to avoid it? My asterisk version is

*0 is the disconnect feature. You should check features.conf for the feature codes currently in use.