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Your LimeSurvey version: LimeSurvey Community Edition   Version 6.3.4+231108 
Own server or LimeSurvey hosting: Own Server
Survey theme/template: Fruity TwentyThree
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I would like to collect the user's phone numbers using a question in my Survey, however for data protection reasons, I don't want it saved with the other answers. The reason I want to collect it, is so that I can redirect users to a url at the end of the survey, where the phone number is a query param. This means, that I would like to have access to the response in the End URL.
Thanks for your help!

You can not have access to something that isn't saved. Sometimes you can save it at the beginning, but then delete it towards the end of the survey via expression manager/script, but in your case this isn't an option either I think, das you need the telephone number at the end URL and afaik there would be no way to delete the number after sending the number via the end URL.

What you could do is redirect the respondent to another script, where they can insert their telephone number then.