A mobile phone service provider wants to survey its customers to study privacy concerns and the sharing of their personal information. They call 5000 randomly selected phone numbers from a database containing the phone number of every customer. If someone selected doesn't answer, they'll attempt calling back up to 2 more times before giving up on reaching that person. They reach 350 customers with this strategy, and 60% of those reached say they are at least "somewhat concerned" about their personal information being shared without their knowledge or consent. which of these is the most concerning potential source of bias?