Anonymous Call Rejection (ACR) blocks incoming calls that have their phone number withheld, and is automatically set by default when registering a new account with us.
If you would like to accept calls from withheld numbers, you can manage this on the numbers section of your online portal here https://my.numberpeople.co.uk/numbers. Click on the number you wish to manage and click on “Reject calls from withheld numbers” to toggle this on or off