Phone numbers will be stored on our servers, as they are required for verification and registration. However, they will be encrypted, and we will not be using them for marketing purposes.
While I expect this to be the case, it will be difficult to verify it. Why not use an e-mail instead? At least people can use throwaway e-mail accounts if they don't fully trust you. There is too many cases of data leaks where information is not encrypted and just stored in plain-text.