I've rolled this out recently as well, we give the user the option of a voice call or SMS. We did this knowing that about 50% of our users sign up by giving us a land line and may not have a cell phone. Twilio provides a code example right on the site for voice based verification where their system will call you and ask you to enter a number you have been presented with on the site. We do the inverse where you get a call/sms with a verification code and then have to enter it on the site.

I would make a couple of points though, we've run into several sites that don't provide a mechanism to have the call retried (answering machine, undelivered cell call, etc) or to go back and enter a verification code if it was captured by voice mail etc. Google lets you enter the code whenever you get it, Twilio lets you initiate a new call with a new number, but many ignore this problem.