They can transfer a call to the voicemail box directly by using the voicemail steering digit + extension. In North America the default steering digit is 7, so if you wanted to transfer the call to extension 100 VM box, transfer the call to 7100.
Your receptionist can do a blind transfer to 7XX, where XX is the extension of the person who is to recieve the voicemail. (I'm assuming you left the default 7XX, check your settings if you purposed 7 for something else other than voicemail.)
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