[asterisk-users] "Reserved" extensions?

[Yuan LIU]

I'm creating extensions _*XX.  But whenever I press *0 or *8, Asterisk 
throws out congestion and hangs up.  I set verbose to 6 and debug to 6, but 
all Asterisk cares to display in console is
    -- Starting simple switch on 'Zap/1-1'
    -- Hungup 'Zap/1-1'

Are these combinations forbidden?

Yuan Liu