I think it means "polite words and phrases." It's honorific language. We usually use it for unknown people or older people.

もしもし　means もしもし(Excuse me Excuse me) means Hollow as you know , but when telephone came in Japan we say 「ＭＯＨＳＵ　ＭＯＨＳＵ」 I tell you, I tel youl) after years it become change the word and word become shortly. then we commonly answer the phone "MOSHIMOSHI" Now.

Other Japanese already answer for it "Keigo" if we say commonly Polite words (language)or respects words (language). You friend mean that words or language. The usage is really complicateed for non- native speaker. The words or sentance should make use differently. Those are classify Imperinal house hold words, repecptive word, polite word, anothers. This is not slang. this is one of Japanese language style.

kei.isomura: thanks, does keigo have another meaning? I was watching this Canadian who lives in Japan vlogging about how hard keigo is and it sounded like she was talking about something like Japanese slang or something, such as using "もしもし” on the phone, can it mean this as well?

It means guard. (protect person or building )