Follow these steps to change which number uses data on your dual SIM iPhone. Only one number at a time can use data. This means, if you are on a call with a line that you have designated as voice-only, your data will not work. You can turn on Cellular Data Switching. This feature automatically turns on the data of the line you are using during a call. If one of the lines does not have data, we recommend turning this feature off, to avoid pay-per-use data charges.