Great question! Although I don’t know if there is a specific function for this on your particular phone, the best way to save the message is the simplest. Turn up the sound as high as you can and record it onto a digital recorder (like you would use for a lecture), a cassette recorder or your cell phone if you have a voice recorder on it. Then save it as an MP3 on to your computer for safe keeping. You can also Google your vtech model number to see if you can find an owner’s manual. They might have a way to save a recording in there. Probably not, but it’s worth a shot.
The process is very simple. You'll just have to click the green phone icon, the same place you make calls, and navigate to the voicemail screen. On the voicemail screen, you'll see a greetings option on the top left.

18. Hello, you’ve reached the voicemail box for [employee name] at [company name]. [Employee first name] has moved onto a different position, but our new [job title, employee name] will be happy to assist you. Please call [number, extension] or leave a message here and a representative will get back to you.
You have reached (Your Name) at (Your Business). I’m sorry that I wasn’t able to get to the phone. If you leave your name and number, I’ll return your call within one business day.