I believe that is a bit odd to do this however I also think that it is appropriate and rather a PRIVILEGE to have the abilities to vote on DDO not a right and showing a phone number is not unreasonable.
There is a good reason to why you must complete three debates on the site to receive voting privileges should you not have a cellular device or a land line. So long as you confirm your age on the site, I do not see it to be any issue.
In all, as long as there is a reliable way to confirm your identity, age, and other necessary things whether by phone, E-mail, or mail I would say keep it how it is.
Now, before you flame me for "Hating on DDO", that's not what I'm trying to do. I just think it is unfair for only those who have access to a mobile phone can have voting privileges. I agree with the "Complete 3 Debates" to avoid easy vote botting, but why the cell phone number? I would love to hear your opinion on this. I'm probably very wrong. If so, please correct me. Thanks. :)