Added search field in Customer list, but user input is disabled

I added the the mobile_no field for searching:
image

It’s showing on my List, but it’s not letting me enter anything:

As you can see from the Inspector, the input is disabled. What’s causing this and how can I solve it?

As a side note, is there anyway to add a search box for all mobile and phone numbers across any associated Contact, not just the primary one?