Dial 9999 on your 3CX phone / App
When prompted enter your PIN # (Your PIN # can be found in your Welcome Email).
Open 3CX app
Press the voicemail button in the lower right hand corner of the screen.
Choose the new message.
Select Voicemail from the sidebar.
Then select how you hear voicemail from the options on the right hand side.
Launch web client (see: How to open web client)
On the People tab choose a person that you want to leave a voicemail message.
Click on the 3 dots and choose leave voicemail.