Ahh - you want to know the "magic trick"...

To park a call from a phone, you invoke the blind transfer (## by default) followed by the parking lot extension (70 by default). In case there are some extra dial plan entries that start with the parking lot extension, you can tell the system that the extension dialing is complete by pressing pound (#) after the extension number.

To configure a programmable key on the Yealink phone that sends calls to the parking lot, you should therefore create a speed dial entry with the digits
##70#
When you press this speed dial button while on call, the phone will dial these DTMF digits - Asterisk will interpret them as a blind transfer to the extension 70 (parking lot) - the call will be parked and the parking lot extension will be announced on the Yealink phone.

To program a button that transfers a call directly to a specific mailbox, you should do pretty much the same - blind transfer (##) followed by voicemail direct dial prefix (#* by default) followed by the extension number (e.g., 221) and finally by pound (#). The speed dial for transfer calls to mailbox 221 would therefore be
###*221#

UCX Guy, I have a Polycom 400 on a UCX50e and want to program a key to send ANY call directly to ANY voicemail box. Please how do I do it?

You could configure a button with digits ## (blind transfer) #* (direct voicemail dial prefix) - that is ###* - the user would have to press this button followed by the mailbox number and finally pound to complete the transfer.