Though cell telephone and traditional landline telephone surveys have many similarities, cell telephone surveys include unique events that must be taken into account when calculating response rates. In addition, events common to both cell telephone and landline surveying may have different meanings (for example, the meaning of a ring-no-answer). Although AAPOR is currently developing response rate standards for cell telephones, they have not yet been released. This paper describes the operational issues of calculating response rates for the National Immunization Survey’s (NIS) Cell Telephone Pilot. The NIS Cell Telephone Pilot is a cell telephone survey composed of approximately 49,300 cell telephone numbers in Illinois. The target population for the NIS Cell Telephone Pilot was mothers or female guardians with children between the ages of 19 and 35 months. The pilot was fielded during the second half of 2007 in preparation for possibly integrating cell telephone interviewing into the main NIS survey. The NIS is a nationwide, list-assisted random digit-dial (RDD) survey conducted by NORC at the University of Chicago for the Centers for Disease Control and Prevention to monitor the vaccination rates of children between the ages of 19 and 35 months. This paper discusses the advantages and disadvantages of calculating cell telephone response rates on a household-level versus person-level. We also discuss the differential meaning of events in a cell versus landline survey, and how final disposition codes are calculated based on those events. Our practical experience is meant to encourage discussion on developing a standardized method for calculating cell telephone response rates.