How do you send voicemail message to another phone?

It depends... Often you can dial the phone number of the phone owning the voicemail box. When it goes to voicemail, press *. That will usually prompt you for the PIN code to access the mailbox.

Some systems will let you dial the voicemail external number. To find that, look in the messaging menu of the phone that owns the account, or in the call history. If that works, you'll probably need to enter * followed by the number of the phone. And then the PIN.