The phrase "What is someone on about?" can be interchangeable with other phrases, depending on the context or region. In the United States, people may use "What are they talking about?" or "What do they mean?" to inquire about someone's communication. In the UK, "What's their game?" or "What's their beef?" may be commonly used to mean the same thing. The phrase can also be replaced with more informal language, such as "What's their deal?" or "What's their problem?" Each expression aims to seek clarification when someone's message is unclear or confusing.