A google voice number would feel less "concrete" to me, since it's tied to a google account rather than a physical sim card/phone.
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.