Landlines and digital cell phones that are registered on a cellular network can be immediately traced. Prepaid phones take a while longer to trace and cannot be pinpointed to an exact location.
Landlines are easily traced via the phone company, whether or not the caller attempts to mask their phone number. Digital cell phones are instantly traceable. In 2006, the Federal Communications Commission required that all cell phone networks use location tracking technology. Simple prepaid cell phones are a bit harder to trace as their location must be triangulated and deduced using cell phone towers. This process does not give a phone's exact location, but renders a close estimate.