I am not receiving calls from a particular external number.

If you enable the feature on a User Phone to "Block callers who are not already in my Contacts list" and also select the default option of "Send to voicemail and directly to spam folder - no notification", calls from unknown numbers will go straight to black-hole voicemail box. To allow a number, go to Dialpad.com, click LOG IN, then Use Dialpad Online. Once the Dialpad user interface displays, click Spam, hover the mouse over the caller you want to unblock and click the drop-down arrow to the right of the time and choose Not Spam.