I think the clean test is whether the borrowed line starts doing narrator work: framing an argument, tightening irony, or setting a pattern the prose then keeps echoing. At that point it is not just a citation anymore, it is part of the book’s machinery. Literary criticism loves to pretend this is mystical, when usually it is just placement and repetition.
An epigraph stays outside the voice because the main text does not have to answer it. A quoted letter or saying can slide inward if the surrounding narration adopts its rhythm or vocabulary, especially when the novel keeps returning to the same phrasing without quotation marks doing much heavy lifting. If you have a specific example, the interesting question is where the shift happens on the page.