I agree with sailing. You really need to decide on which service to use.
They all have pros and cons.
You didn't really say if the service you decide on had to be free. If so, Google Voice would be your choice.
Google Voice is quite nice unless you require support services and then you're stuck with only their online help forum for any support you need.
I decided to pay a few dollars a month and went with
Voip.ms as my main provider. I have tested others as well, cause their all so darn cheap it's easy to do.
A lot of good providers have been mentioned throughout this forum, such as, Callcentric, Anveo, Future Nine, and many others.
Do some research on these companies and find out which ones might fit your needs.
Then pick one or two and test them out on your Obi.
Good luck.......................
Forgot to mention: Voip.ms is offering free porting at the moment, don't know if any of the others are.
Also, if you need 911 service you would need to choose something other than GV, as they do not provide it.
Of course if you do need 911, you could put GV on as SP1 without any 911 service and add one of the other providers to fill the 911 need on as SP2.
Most providers that offer 911 are in the $1.50/mo. range, Anveo is a few pennies cheaper.
Again, good luck.......