Do you keep data that's not up to date, like former clients, perhaps?

If so, you will likely eventually find phone numbers being re-used, either quickly, or slowly.

Also, wouldn't it be possible for a small client (start-up) to use the same phone number for more than one person (two people sharing a desk and phone)?

What about very large clients where you call the central switchboard/front desk, and then have to ask for the person you want?