I think Ooma is still working on a roll out to everyone from last I saw, I am not sure if its out to everyone yet or not, I know they were working on it, but then held off a bit to fix some bugs.
There is no way to know on your end if its enabled or not. This would be a nice addition to the setup.ooma.com site, click on status and have it tell you if PureVoice is working or not. I knot this probably would be difficult of not impossible, but may be helpful to some people.
Otherwise go to the Ooma Status and updates forum and check the latest firmware, then go to setup.ooma.com and click on status, and at the top it will give you a REV number and make sure you have the latest firmware.
Cox / Ooma Telo / Apple AEBS (Time Capsule) / Ooma Handset / Premier
CompTia A+ CE
MCITP: Windows 7 Enterprise