When editing numbers individually, you're able to go into the configurations and add phone numbers to be notified upon a missed call/voicemail.
To get to the configurations, enter the number into the search box in the Call Center.
Once the configurations page loads, scroll down to "Notification Phone Numbers" as seen below.
After adding the number you would like, press the enter key to add the number into the configuration.
You are able to add multiple numbers to be notified.