This applies to to Polycom VVX phones:

1. Navigate to 'Directory'
2. Select 'Contact Directory'
3. Select 'Add'
   1. add a name
4. Select 'Add contact'
   1. mode -> Tel
   2. Telephone number must be entered exactly as the number appears when dialed in. (i.e. press star twice for '+', '15556667777') 
5. Select 'Auto Reject' -> Enable
6. Select 'Save'