Automated customer service surveys (via e-mail, web, or IVR) have one huge virtue: they are cheap. Big companies can afford to collect feedback from millions of customers, creating a broad statistical profile which can be used to measure customer satisfaction and other key metrics with a lot of granularity. However, automated surveys often suffer from poor response rates, and can usually only have a handful of questions before customers start abandoning the survey. That severely limits the quality and depth of the feedback.
Phone interviews, on the other hand, are almost exactly the opposite: customers are much more willing to participate when there's a live person on the other end, and they are willing to answer a lot more questions as long as the interview is well-structured. Interviewers can also probe and go in depth in a way a computer can't. But it's inherently more expensive to conduct a live interview, and there's just no way to escape that fact.
In some companies I advocate for a hybrid approach, combining the vast ocean of customer feedback affordable with an e-mail or IVR survey, with the depth and reliability only attainable through interviews.
This takes advantage of the fact that in most companies, most customers have a reasonable experience most of the time. If a customer was satisfied with the experience, there's usually little to be learned about how to improve things. On the other hand, if the customer was not satisfied you can get a lot of useful information. Running both an automated survey and an interview process in parallel focuses the survey dollars and effort where it is most effective.
The automated survey is used to establish high-level statistics across the organization and identify underperforming segments. The live interviews are used to generate actionable feedback about how to improve. Customers are selected for one survey or the other based on whether the statistics suggest a high probability of dissatisfaction.
The end result combines the best of both worlds: tons of high-level statistics across the entire organization, and highly detailed in-depth feedback where the company needs it most, all delivered in a coherent package to the people who need to make both strategic and tactical decisions about how to deliver the customer experience.