Sometimes doctors conduct research about their results by sending patients surveys or asking questions over the phone. The person calling is often an independent worker who doesn't work for the clinic. This helps keep the research pure and unbiased.
Researchers may send patients a letter ahead of time letting them know what's coming and what to expect. It's easy to read such a letter and toss it out without realizing what it really means. When the call comes, the patient is surprised or doesn't remember anything about it.
It's always a good idea to take the name and number of anyone asking personal question. Then you can call that person back. You can also ask for the contact information of the person in charge of the study.
Doctors rely on patients to help advance medical research. Your cooperation is important, but the decision is yours entirely. You can always decide not to participate.