Complex sentence: Most of the time, these companies fake the caller-id tag, but sometimes the number is real, and when it is called back, it plays an automated message selling the call blocking service which promises to stop unwanted calls.
Split into simpler sentences: Most of the time, these companies fake the caller-id tag, but sometimes the number is real. Sometimes when it is called back, it plays an automated message selling the call blocking service which promises to stop unwanted calls.