Call scheduling is a challenge for surveys around the world. Unlike cross-sectional surveys, panel surveys can use information from prior waves to enhance call-scheduling algorithms. Past observational studies showed the benefit of calling panel cases at times that had been successful in the past. This article is the first to experimentally assign panel cases to previously beneficial call windows. The results from a large-scale national survey in Germany show modest efficiency gains measured in number of call attempts needed until first contact but no gains in efficiency to gain cooperation.