A low-ball estimate (or quote) is when a shop intentionally quotes a job at an unrealistically low cost. Some shops do this to get the customer to agree to the work but then call later and say more money will be needed.
A phone call telling you that more money is needed is not an automatic indicator of dishonesty. Modern vehicles are complex and unforeseen problems will sometimes arise. However, a shop should never quote you a low number to get you in the door, knowing that they’ll try to talk you into spending more money after they have your vehicle.
When not accounted for in the estimate, the customer will end up paying more than they expected and planned for.