Ah, the plot thickens like a hearty soup simmering on the stove! This might happen if a business or individual changes numbers, but you're still using the old one or if there's a glitch in the phone network. It's also possible that the number has been deallocated but has yet to be reallocated. So, it's not you—it's the number's complicated life.
Tip: Double-check the number, dial again, and if the message persists, contact your service provider.