Ask HN: Dealing with nuisance phone calls

We are a small business that has a number so we can receive calls from clients. Unfortunately, recently we've getting about one call a day from people who either seem to be phishing (by asking us for personal details) or are trying to sell us investment plans.

Each of these calls only takes a couple minutes to deal with. However, it is rather distracting. Furthermore, because we now assume whenever we get a call it will be one of these, we are worried we'll get a false positive and miss and potential clients.

Has anyone else had this problem? Any suggestions about dealing with it?