Why can't I add "Home Phone" to a Case?

Why can't I add "Home Phone" to a Case?

I am using Cases to record technical problems logged by Contacts. Some are at Home (Residential) and others are at Work. Under Contacts i record Home Phone or Phone (which is work phone) When I log a Case , there is only an option for Phone (Work Phone)
Even in Customization - Layout, there is no option to add in Home Phone to a Case. I even upgraded to Pro version trial but no joy.
I am left with the only option of re-importing records and pointing Home Phone records to the Zoho "Phone(Work Phone) field during the Import process. Am I doing something wrong?!