Most people are not finished when you think they are, and the last part is usually the useful part: the real concern, the actual number, the thing they were working up to. Interrupting removes it. Letting someone run to the end costs a few seconds and often hands you exactly the information you were planning to guess at.

In the next conversation that matters, resolve not to speak until they have clearly stopped. Then ask one more question before you respond. People tell you what they want if nobody cuts across them, and knowing it leaves you better placed than talking would have.