How do you handle the Phone Bill that you get at one subsidiary but wants to charge 50% to the other subsidiary?

Intercompany Journal entry. There are products out there (Shared Vendor Bill from NS, Nerdly Multico automation) that will create intercos for you when saving the bill, but they'd likely only be worth it if you're doing a bunch of these a month. If you're only doing 1, it would be more trouble than it's worth.