For the Grandstream phones in the office, here are the steps to complete a transfer to voicemail:
While on a call, press the Transfer Button once, the system defaults to an Attended Transfer (AttTrnf). The Attended Transfer requires that the party answer the line before you can transfer the call. You will need to switch to a Blind Transfer (BlindTrnf) by pressing the button shown below:
Dial *4(followed by the extension) and press the BlindTrnf button one more time. The caller will be instantly transferred to the voicemail for the extension dialed and the call will end on your phone.