Most extensive tests are necessary if the system has been developed for a specific user. In this case the user should test all functions. For commercial off-the-shelf systems that come with a validation certificate, only tests should be done of functions that are highly critical for the operation or that can be influenced by the environment. Examples are data acquisition over relatively long distance from analytical instruments at high acquisition rate. Specific user configurations should also be tested, for example correct settings of IP addresses of network devices should be verified through connectivity testing.
After the security question, start with general questions. If you want to limit the survey to users of a particular product, you may want to disguise the qualifying product. As a rule, start from general attitudes to the class of products, through brand awareness, purchase patterns, specific product usage to questions on specific problems (., work from "What types of coffee have you bought in the last three months" to "Do you recall seeing a special offer on your last purchase of Brand X coffee?"). If possible put the most important questions into the first half of the survey. If a person gives up half way through, at least you have the most important information.
After a telephone call, the interviewer selects a result (Complete, No Answer, Busy and so on). They can make an appointment to call someone back at a specific date and time and can leave comments about call attempts. You can restrict which interviewers can handle certain types of calls, based either on the results of previous call attempts or on prior information you have about respondents. You can also determine results automatically, based on answers to survey questions. You can schedule calls by time zones.