There is no way to know who has listened to your voicemail unless the caller leaves you a message.
With Visual Voicemail, you can see a list of your messages and choose which ones to listen to or delete. Learn how to set up and transfer your existing voicemail. Create a voicemail password, then tap Done. Enter your password again to confirm it, then tap Done. Select Custom or Default. If you choose Custom, you can record a new greeting. Go to the Phone app, then tap the Voicemail tab. Tap Set Up Now. Enter your password,* then tap done. Record a custom greeting or use the default greeting, then tap Save.

There is a limit on the VM length but you can modify it. This line of the script has a maxLength parameter, represented in seconds: resp.record(maxLength=”30″, action=”/handle-recording”). If you changed that value from 30 to 120, for example, the max VM recording length would be 120 seconds, for example. Note that the voice mail recording is not stored on the Pi; it’s stored on Twilio’s servers, hence the per minute charge from Twilio for the recording and the service. Is it possible to use it as CCF (conditional call forwarding)? after enabling the forwarding option it should play some pre recorded messages on the basis of who calls ?
To upload a file, click the upload button. Next click browse and select the media file from its location on your computer. Click the blue upward arrow icon to begin the upload. When you are finished, click Save Changes. Next, click on Advanced and select Options. Please make sure to check the box in front of Already Setup. Next add recipients by clicking on Advanced and select Recipients. Please note, the voicemail box must have a greeting associated with it before you can add recipients. Recipients allows you to email selected people when the voicemail box gets a message. For User voicemail boxes, the email addresses you add here are in addition to the user’s email address given at signup.