From time to time you'll call back a lead that called your practice using a trackable phone number, and the answering person will say they never called you. If you see a pattern of this happening, it's a sign that these calls might be spam. 

These are spammers using a technique known as 'masking.' They co-opt a real number similar to the number they are calling as a mask for their own caller ID. So for example if my number is 555-222-3333 they will mask their spam calls to me from numbers starting w/ 555-222. They are real numbers, though, so when you call them back you get a person that never called you.

Masking happens to people on any providers (AT&T, etc.) Our provider is Twillio and we regularly talk to them about the spam our customers receive so they can try to find ways to block it.

If you know a lead is spam, you can Disqualify it.  Also see the related articles below about spam.