As described in A more advanced overlay - Advertisers it is possible to create predefined groups for your overlays. Creating advertisers in advance corresponding the sources you would like to track, and by specifying which advertiser a particular overlay belongs to you can streamline the process of filtering and grouping call statistics. Advertisers can be created and managed through the stats area, however if you regularly need to create or edit advertisers you may wish to use the API functions to do so.
Note, the word advertiser is something of a misnomer. Originally it was used to identify which website an overlay was shown on, but it can be used for any purpose. Common uses include search terms, referring websites and software versions. - Anything for which you are curious about the breakdown of your call volume!