I've seen sites where you have to activate your membership using a phone. Basically what happens is they either send you a text message or a computer calls your home phone telling you the activation code.
I've had problems on my site where spamming users create many accounts on my site only to use them to spam but if I put a restriction like this on my registration, I'd have less problems like this.
So my question is, how can I make Phone Activation a requirement for my site? Are there any services or scripts on the internet that do this?