Hey @julia n​ if the number is saved as custom attribute that means you can search users with that attribute value, and then check conversations around the time you know this conversation was conducted. I understand this can be a bit hard if the user had many conversations. But anyhow, custom attribute can not be overwritten with custom bot (it can be set only once as far as I know), so if your users are making multiple requests of that type, you should stay away from using custom attributes.
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Or you just let them type in the number in composer and then it will be searchable (and perhaps an agent manually creates a tag with that number).