Excel doesn't have built-in functionality to directly customize chart drop lines for callout emphasis. However, you can achieve a similar effect using a combination of chart elements:

Here's one approach:

1. Create your chart: Start by generating the chart you want to customize.

2. Add data labels: Data labels display the actual data point values next to each point on the chart. You can use them as callout boxes.

3. Format data labels: Right-click on a data point and choose "Format Data Labels." In the formatting options, you can:
- Increase the font size and change the font color for better visibility.
- Adjust the position of the data label relative to the data point. You can choose options like "Above", "Below", "Center", etc.
- Change the data label fill color and border for emphasis. Consider using a semi-transparent fill to avoid obscuring the chart.

4. Add a trendline (optional): If you want to emphasize a trend, you can add a trendline to your chart. Format the trendline with a distinct color and style to make it visually prominent.

5. Format chart elements: Customize the chart elements like chart area, axes, and gridlines for a cleaner look.

Alternative Approach (Chart annotations):

1. Create your chart: Similar to the previous method.

2. Insert a text box: Go to the "Insert" tab and choose "Text Box" from the "Illustrations" group. Click on the chart area to insert a text box.

3. Format the text box: Type your desired callout text inside the text box. You can format the font, size, and color to match your data labels (if used).

4. Position the text box: Drag the text box to the desired location near the data point you want to emphasize. You can also add an arrow to the text box using the "Shapes" functionality for a true callout effect.

5. Format chart elements: Similar to the previous method.

By using these techniques, you can create custom callouts to highlight specific data points within your Excel chart, enhancing readability and visual communication.