Warm transfer to voicemail using the Cisco 8861 phone

Check this link on how to transfer a call using a cisco phones.

https://support.ringcentral.com/s/article/Transfer-Call-to-Voicemail-Cisco-Device?language=en_US

Transfer to Voicemail – Cisco Phones

1. During the call, press the “BlindXfer” soft key. Blind transfers only show the originating Caller ID.

NOTE:  It may also be labeled as “bxfer” depending on the specific set type.

2. Press *0.

3. Enter the extension number of the voicemail that the caller is to be sent to.

NOTE:  This can also be your own extension number.

4.Press the “Dial” soft key. 

The caller is then immediately sent to the voicemail box of the destination extension number you entered.

I believe the only way to do the warm transfer to a voicemail would be to complete the transfer just like you would if you were connecting them and then press pound (#) on the first ring.

During the call > Press Transfer (depending on your default you may have to press "Consult" soft key to switch to the warm transfer option) > Dial the ext > Press pound (#) on first ring > Press transfer soft key

EDIT: Problem with this is that the caller may miss the first initial "Hey you have reached the voicemail off.." line because you are not quick enough to hit the transfer button.