[HELP] Setting Message Waiting programmatically

I have found that Asterisk’s internal Voicemail can set & reset the MWI lamp on my SIP hardphones (Avaya 4602’s).

Is there a command within Asterisk to do this manually, without using voicmail?

I ask as i have hooked up an external voicemail system to my asterisk box, but cannot work out how to notify of new messages