interrupting phone calls -- why?

Some people may think it's okay to be interrupted while they're on the phone -- maybe their calls aren't very important or they don't consider the people they're talking to very important? -- but if I'm at work, and I'm on the phone, especially when I'm working at a public desk, I can guarantee you that my phone call is nothing but business and I'm doing my best to treat the person on the other end with as much respect and consideration as I would any person that walked up to the desk when I wasn't on the phone. Why would anyone expect me to act otherwise? I really don't understand staff and patrons who think it's perfectly okay to interrupt a phone call.