Telco and Comcast probably are the most secure products. Skype and Ooma seem to have worked a bit on security, the problem is the industry hasn't. Most other VOIP providers don't currently support any encryption.
The area of internet where Ooma calls could be intercepted is between the Ooma servers and the Ooma PSTN carrier. If I understand correctly this is usually a large pipe with many aggregated calls. If it is hacked, it's not only a problem for you but for many others.
Now, go have some fun and open that box.
Once you have an Ooma phone number (in about 10-15 minutes after you open the box) you'll be able to get an Ooma forum account and ask questions of the Ooma staff online.
Good luck!