I had this problem for a long time, which can be embarrassing doing phone support, especially one with frequent callers that know my name and voice (one of only two men and we have distinct voices and greetings). I started intentionally using callers name's three times in every call and reaped several benefits: 1) I actually remember their names when they call back, 2) I'm better at remembering names having been told only once (even outside of work), and 3) my customer satisfaction scores had a marked and sustained increase.