BT (LSE:BT.A) shares have been one of the more surprising performers on the FTSE 100 over the last couple of years. The telecoms stock has climbed over 80% since May 2024 as new chief executive Allison Kirkby makes real progress on the company's long-running restructuring. And yet even after that impressive run, the shares still offer a 4.3% dividend yield today, nearly 50% more than the 3% offered by a typical FTSE 100 index fund. So is this a top-notch income stock, or is there a catch? What's holding BT shares back?