I just want to second that. I work closely with Voxer (I'm at Joyent), and these guys are both very sharp and dealing with some very interesting problems. Add in the kicker that this is a commercially relevant problem (I am an avid Voxer user as well), and I don't know what else you could ask for in a startup. For whatever it's worth, I worked with Twitter in a similar fashion way back in the day [1], and I can say that Voxer is every bit as exciting and promising -- but with a much more technically talented team. If you're a whole-stack software engineer in San Francisco, you owe it to yourself to check these guys out!

[1] http://www.redmonk.com/jgovernor/2007/04/24/twitter-is-going...