Specific interviewer characteristics, interviewer continuity, or matching interviewer and household characteristics may increase cooperation, especially for difficult-to-convince households. In face-to-face surveys, unobserved heterogeneity often makes a proper analysis of interviewer effects impossible. Although surveys conducted in telephone centers usually assign households to interviewers at random, there is less research on interviewer effects on cooperation, probably because telephone surveys produce smaller effects. Using data from a large telephone panel survey, I find interviewer effects only for households that refused to participate in a previous wave. Interviewer continuity or matching interviewers and households on sociodemographic variables has weak effects for any type of household. Interviewer experience has positive effects for previously refusing households only. Telephone survey organizations therefore only need to worry about using specially trained interviewers for refusal conversion calls, while specific assignments of interviewers to households are not necessary.