From the leads they already paid for. Every business you run ads for is sitting on a pile of old enquiries and quiet quotes. Recover those and it goes on your invoice, not your delivery team. Every client account is set up with you on a forty-five minute call, included, and the number stays theirs.
One line
added to an invoice you already send
You are already in the account. Recovery is a service you can add to the next invoice without hiring anybody or running another campaign.
No new ad spend
the leads are already paid for
The leads exist and the client paid for them once. Nothing here competes with the ad budget you are already managing.
Booked work
not another lead report
Show appointments recovered from the client's own list. That is a harder thing to churn than a monthly report of leads delivered.
Twenty minutes, and you will know whether it is worth running for your clients.