How to prevent users from circumventing a form?

I have a multi step form which starts with a question. If the user says No, they are taken to a screen instructing them to call in (since the system cannot process No answers). If the users say Yes, they are taken to the next step of the form.

The problem is that many users, after selecting No and seeing they have to call in, will simply go back to the question and say Yes to proceed to the form.

This creates a lot of issues down the line since we cannot service No customers (therefore we would have to redirect them, etc), creating a lot of service cost.

There is no way for us to discriminate Yes vs No users before them accessing the form to complete.

What are some solutions which can alleviate this issue?