To be able to filter calls in your call log by a category, add a "tag" to the call in Slack.
Tags can be created right in the Slack interface and can be named individually.
Add as many tags that are necessary to filter your calls when needed. Tags can either be a word, multiple words or a emoji. 

How to tag calls: 

Start creating a tag by going to a call that is either active or has ended and start typing in the field that states "Tags". 

Type the word or category that you want the call to be listed in later in the Call Log.
If a tag does not exist yet, Yodel will let you to create a tag. 

Yodel will show you the available tags in a drop down menu if a tag already exists.

After you create one or more tags, the drop-down menu will display all tags that have been created and used for at least one call in the past. 

Is a tag active, it will be displayed underneath the information about the call length. 

How to un-tag calls: 

Un-tag calls by clicking on the category and the drop-down menu will appear again. Click on the category that is selected and the call will be un-tagged from the previous selection. 

How to filter calls through tags: 

Go to your Yodel interface and select on the left side the section "Call Log".
Filter calls by typing the tag into the field for tags and click "Filter". 

All calls with the entered Tag will be displayed in the Call Log and can be downloaded.