Unfortunately we do not query for call information for that report. You will have to add a custom command and report to query for that data as well.
You would probably have to create a QueryClass element for that particular report, copy it and override the Request component to query the Calls information (implementing RequestedDataSetMixin, IFilterDataRequest, IParameterDataRequest interfaces) . Only then you can then add the call attribute in the IncidentReport.xslt as you mentioned.
It is recommended that you create a new copy of the IncidentReport.xslt for your new report.
Would you be so kind as to update this post by marking one of the answers as correct?