I use a 22 Hornet for most of my predator calling, especially at night. It's light, handy, cheap to reload, and has plenty of killing power at normal calling ranges. It's also easier on hides than the faster/bigger calibers.
I've thought seriously about building a 9MM semi-auto rifle(scoped) for foxes, but I've never done it yet specifically because I know it'd be a little to weak for coyotes. Around here, you're just as likely to call in a coyote on a fox stand, and I'd feel undergunned with the 9MM.
On the other hand, I've used a 357 rifle(perfect for daytime calling), and it's a dream on a calling stand. I shot mostly 38 Specials out of my last 357 rifle, and I can say for sure that the 38's from a rifle are plenty strong enough for coyotes on a stand.