This may be worth speaking with our Product Helpdesk Team for the BT8610. You can find their details on the below link:
Can anyone clarify why an ex-directory caller who announces themselves is sent direct to the 8610 answer machine? I don’t have an option to accept the call etc. Thanks
Although ex-directory doesn't mean a caller number is withheld, are the ones you're getting withholding their number - and you've set the 8610 to make all those calls go to answer service?