Note

The following method is optimal:

1. Pick up the second customer's phone.
2. Pick up the third customer's phone.
3. Move to a point $7.5$ meters east of the Startup Hanbyeol office, then return and return the third customer's phone.
4. Return the second customer's phone.
5. Pick up the first customer's phone.
6. Move to a point $10.5$ meters west of the Startup Hanbyeol office, then return and return the first customer's phone.
7. Move to the Startup Hanbyeol office.