You may not be able to forward your messages as Boost does not support Visual Voicemail (ios), although it is something that Telstra offers for their Post-Paid customers it is not available on Pre-paid for either company.
Some phones allow you to 'Save' you messages to your device, however if it is an available feature, when you listen to your messages, you may be presented with a menu after you have listened to the message. If you have the ability to forward the message the option will appear there.
Otherwise, if you MUST have this message on another device, you can play the message on a speaker (bluetooth or use an aux cord) and then physically record it on the second devices audio recording app (most phones come with a voice recorder as standard)
You could also look at your app store for any 3rd party apps that may allow you to do this.