But I will never provide my mobile number. I will not "use a friends phone" or "buy a disposable phone" as all the workarounds seem to suggest.

You know what I did instead? I signed up for gmail. As providing a second email is perfectly satisfactory to meet your supposed conditions for requiring a mobile number, I went with that option.

I understand your need to strip mine personal information as part of your business model, but you are misjudging the market. Most subscriber rate surveys and trend reports show a clear downward trend in your share of new subscribers since 2010. For the sake of your own jobs, you might want to review your policy.

Remember AOL? They misjudged the market too.