For me, it wasn't the cell phone number request that got me suspicious, it was the fact that they required me to tell them who my cell phone CARRIER was as well. I couldn't think of a valid reason they would need to know who my cell phone provider is, so I backed out. But as you said, by that time they had my email address and some other information. And that was the beginning of two a day emails trying to get me to finish my registration and send a link to everyone I know.

That part is very suspicious.