If this is talking about forums in general, then I'll never give away my personal data, regardless of how trusted a website is. I'd never register an account on a forum that requires a mandatory phone number to begin with. If you're concerned about signature campaigns, I don't think legitimate ones require you to use an account that has personal information either. Most of them don't allow alts, but if you stick with one account it should be fine. Just ask the campaign manager if you need more clarity. You should be suspicious if they demand your real name, phone details, etc. As for e-mails I also use throwaway e-mails unless you're dealing with a service like exchange or something similar. CMIIW.