Expect the plumber to clear the line if it's blocked, then push a camera from the cleanout toward the tank to find the defect and measure how far out it is. A locator on the surface marks the spot so any digging stays small and targeted.
Depending on where the break is, the repair may fall under a plumbing permit from the Building Department, the Board of Health's septic rules, or both, so ask the plumber which applies. Plumbing applications go in by mail or in person, and work at the tank, the leach field, or a cesspool may call for a Title 5 inspection or a Disposal Works Construction Permit.
Duxbury adopted acre zoning after Route 3 arrived in the early 1960s, so many houses sit on large lots. Ask how far the tank is from the foundation, since a longer line means more yard to dig or line, and ask whether trenchless lining or bursting suits your pipe.