If you have a VOIP plan, you'll be able to utilize voice mail for inbound numbers.
1) Record your messages as mp3 files and save to your computer or call the number in Radius to record that way..
2) In Radius, Go to Settings> Phone Dialer & Call Queue Lists> Voicemail Greetings> Upload Greeting
Or Call the number you see in your Voicemail page to record the message. Or use the Text To Voice option for an artificial voice to speak for you.
Name them uniquely, such as Main VM, Attempt 1 VM, Attempt 2 VM, Quote Sent VM, etc
3) Go to Inbound Numbers
4) Edit the number you want to assign the VM to (note you have to do the phone number verification first).
Select from the greetings in the dropdown which message should be attached to that phone number. You can select a Welcome Message for when they call before being connected with an agent if you want a "This Call May Be Recorded..." type message and you'll select what VM they'll get if you are unavailable to take the call.