I like the explanation that devious spirits cannot say this phrase and that's why it's used. Apparently it's also just a casual way of saying "I'm ready to talk" and was used by early telephone operators in Japan. It's most likely people just ended up copying the phrase from operators and aren't worried about being tricked by foxes.

Japanese people answer their phone like that, everything is fine.

I answer my phone like that, I'm branded a weaboo for the rest of my life.

I can't read this phrase anymore without reading it in Admiral Kizaru's voice automatically.