It recently came to my attention that you can access mobile phone numbers in http request headers but only for specific ISPs/Cellular Data providers.
Apparently you can only access this MSISDN header if the user is connected to the internet through Cellular, not WiFi.
Any idea of which providers support sending this header?
Last time I was directly involved in this sort of thing, carriers would only provide it to whitelisted IPs (essentially partners of theirs that need the data for some reason) or over VPN connections with them. Providing this to just any site would be quite a violation of privacy. I would be very interested if you are able to track down any carriers that are sending this out on all requests.