Need a dead simple VOIP option for someone that just wants to make calls on their cell -- is Ooma it?

[clay]

I'm helping someone that would like to have a separate number from their personal cell. They'd like to be able to publish this number, people call it, and it rings on their cell phone. And then they'd like to be able to use their cell phone to make outgoing calls that appear to come from this published number.

How seamlessly does this sort of thing work when using the Ooma app on an iPhone or Android? Is there a different solution that would work similarly well without being too complex for a very basic user?