@Ay
If it's from a mobile that they call you from, you might be able to drop them a text or phone back. No idea if it comes through as withheld though. That said pretty sure all you need is someone in the house.
Have a nosy at this link. More importantly the section re 'Do I need to be home when my broadband goes live?'
https://help.nowtv.com/article/i-ve-ordered-now-broadband-what-happens-next#
If you need to call the broadband team, when you click 'Get in touch' you want to select the following;
- NOW broadband and calls
- Installation and setup
- Then click the 'How to get in touch' drop down when signed in and this will give you the phone number to call
FYI that I do not work for NOW, just a NOW customer trying to help
