order by field name (other call number)

I am trying to order my books by "Other Call Number System" which I use for location. I can do that by clicking on the column's header, and that works. However, I want to sub-order on author name. For this combination I need to click the up/down icon in the menu bar. However, "Other Call Number System" is not an option on which to order. Basically every other field is an option here, just not this one.
Am I missing something here?

Click first on your suborder, and then on your main order.

The order already set moves into second place and the newly set order becomes the main order. This only works for two orders.

When we finally got Other Call Number it did not make it into every place where it should be.

Thank you MarthaJeanne!
With some additional help I was able to do this- and it does what I need. :)
Thanks,
Remko