For cell/mobile phone, I think we use "핸드폰" or "폰" in most cases. "전화기" means "telephone" or "telephone device", so it's more natural to use "핸드폰" or "폰". Ex) 나중에 내 폰으로 전화해~ = Call me on my cell later~ 내 핸드폰 못봤어? = Have you seen my cell phone? Hope it was helpful!