Change voicemail default voices

I used voicemail for my IVR. I added 6001 as default voicemail Extension. After that, when user click the keep a voice mail, bellow sounds play before keep the voice mail.

 -- Executing [2@agent_busy:5] VoiceMail("PJSIP/my_sip_trunk-00000000", "6001@default,u") in new stack
    -- <PJSIP/my_sip_trunk-00000000> Playing 'vm-theperson.gsm' (language 'en')
    -- <PJSIP/my_sip_trunk-00000000> Playing 'digits/6.gsm' (language 'en')
    -- <PJSIP/my_sip_trunk-00000000> Playing 'digits/0.gsm' (language 'en')
    -- <PJSIP/my_sip_trunk-00000000> Playing 'digits/0.gsm' (language 'en')
    -- <PJSIP/my_sip_trunk-00000000> Playing 'digits/1.gsm' (language 'en')
    -- <PJSIP/my_sip_trunk-00000000> Playing 'vm-isunavail.gsm' (language 'en')
    -- <PJSIP/my_sip_trunk-00000000> Playing 'vm-intro.gsm' (language 'en')
    -- <PJSIP/my_sip_trunk-00000000> Playing 'beep.gsm' (language 'en')

But I need only keep “vm-isunavail.gsm”, “vm-intro.gsm” and “beep.gsm” only.

How do it.?

Try:

cp /var/lib/asterisk/sounds/en/vm-isunavail.gsm /var/spool/asterisk/voicemail/default/6001/greet.gsm