Unfortunately, Limesurvey doesn't offer such a browser switch yet. The workaround is to simply create a copy of your survey and assign that copy the iphone optimized template.
You can also put a link on the welcome screen of the base survey, directing the smartphone users to that special version of the survey.
Furthermore, it makes sense to run the smartphone survey in question by question mode so users don't need to scroll that much.
Later you can easily merge all results at one survey using Limesurvey's VV export/import feature:
docs.limesurvey.org/Exporting+results&st...Export_and_VVImport_