Looking at the photos it doesn't seem there is a lot of privacy unless you permanently camp out in a conference room. I'm on the phone pretty much all the time, would this not be a good fit, for when I'm in SF?
However, there are also several hackerspaces that have woodshops and whatnot, so it seems like a short hop to think that if computery people can work as part of a multidisciplinary office next to a bandsaw, why not work alongside daycare workers? Maybe there are some legal issues in sharing in this way, but by splitting a big open office into a daycare side of a wall and an internet one, maybe there's a way.
However, I believe some of the other co-working spaces in SF (including the other NextSpace ones further into the city) have multiple phone booths so you may want to check those out. Drop-in cost for most spaces tends to be about $20 - $25/day.