LISTCat looks at the system catalog. The volume information you want is held in the VTOC (volume table of contents). Presumably this is linked to your other post about running it continuously as a background job.
You can get the VTOC free space from panel 3.4 in ISPF. It must get it from somewhere using an ISPF service, but I can't easily see which one from the Services Guide. Does anyone else know where it obtains the information?