Discovery calls are the initial stage in a selling process. I am biased but "selling" to a customer provides a much more honest picture of the things they truly care about then a generic customer interview. Despite most folks aversion to sales, if you have a compelling story and can narrow in to a general area of pain (which you have already) most people are very willing to hop on a call and tell you exactly the things you need to know. Be sincere, open to their expertise but be willing to challenge and ask prodding questions. This helps test their conviction and set the weights of what will get them to influence internally to purchase - the true kpi of most products. You might feel goofy since they you don't have anything to truly sell yet. Rest assured, your buyers will be very willing to cooperate if you deliver and you will stand out from the crowded majority of those who don't.