He do give his Google Voice number, it's literally a phone number. He said that he is more comfortable with that because he can turn off the number whenever he want. This is exactly what another cellphone would do.
This seems like an insane way to vet potential developers, by the way. Someone who bases a hiring decision on what brand of phone service your employee/contractor has chosen is probably not looking at the right metrics.
You can take all sorts of little clues from these kinds of things. For example if a website is phrased and designed to look corporate but the only number published is a mobile and the email address ends in @hotmail.com then you know you might be dealing with a single freelancer.
I have a Google Voice number that used to belong to a real actual cell phone.