How do you transfer a call to someone's voicemail?

How do you transfer a call to someone’s voicemail?  In this case, a secretary has her boss’ extension programmed as a soft key on her phone so she can answer it.  A call comes in on her boss’ like, she answers, and would like to transfer it to her boss’ voicemail.  Nothing I’ve found online seems to work (I’ve read dial *9 + extension, dial **9 + extension - neither work)….  Any suggestions?