In the script builder, a Condition node routes the call without asking the agent anything. A condition can branch on the lead's list membership: choose a lead list per branch, and the lead follows the first branch whose list they belong to.
How to Set It Up
Open the script in the V2 builder and add a question, then set its Type to Condition.
For each branch, choose the lead list it should match.
Order the branches. Branch order is the priority, so the first matching branch wins when a lead sits on several lists.
A Condition can also jump into another script.
Example
Put your book-of-business list on the first branch and a general list on the second. A lead on both follows the first branch, so existing customers get the path you want before the general fallback.