Caller bypasses Caller ID notification

I have my landline phone number ported to Zen Digital Voice. (on a Zen full fibre service)

It all works very well with one annoying exception.

My BT DECT phone has a do-not-disturb facility based on the incoming Caller ID. The phone is set to igore any incoming calls except those in my VIP List.

This works for most unwanted calls. But, ever now and again an unwanted call gets through.

I wonder if this might be because the caller ID is being delayed until the phone is already ringing. Is this possible? And, if so, is there any way to stop it?

What is the phone set to do if there is no caller id?

In reply to a post by DFScale:

What is the phone set to do if there is no caller id?

Thank You. That may be the answer.

The phone is now set to block calls with no caller ID.