I’m supposed to work part time, but the load is full time

Question: “I work in a small office of 10 employees (an insurance company).  I work 9 a.m. to 2 p.m.; all the other employees work full time. At times I need to pass work to others to handle in the afternoon hours. When I take the issues to the supervisor, she directs me to distribute the work to other employees, which they don’t appreciate. I get that. But my absence in the afternoon means that others are fielding my return customer calls. Regarding accountability and productivity, I feel that my boss and owner want to see completion of a full-time work load in 62.5% time. Any advice on the best way to handle this? —B.B.