b, you’ve brought up an interesting point. How does signing up for FR on a smart phone effect ip addresses, I wonder? Hmmm...
As with any wired host, a cell phone exposes limited information to Internet servers in the form of a User-Agent tag and an IP address.
...cell phones do provide an IP address. I'd expect that address would come from the service provider, so it may be static, but that's a big assumption on my part. Don't have the time to delve into this further today, but there may be some worthwhile stuff in the document, or other links from the Google search.