Just set up Ooma and use it to make calls.
When you decide to port your current number, then your existing service will go away and you'll receive calls on Ooma as well.
Connect the Ooma, plug a single phone into the Ooma unit itself.
Once porting was done I simply disconnected my Fios phone line (because there was still a dial tone being fed into my line from Verizon). Disconnect was simple, open access cover on box outside of house, in the case of my Fios there were two wires (blue & blue/white) connected to two push to release type terminals. These fed the phone lines within my house. Once disconnected the Fios dial tone was gone. It did not affect the internet since that got fed in to the house via coax cable.
Now plug the wire that you are using to feed the one phone you have plugged into your Ooma into any convenient phone jack and it will feed the lines within the house giving you Ooma service everywhere you have a phone jack now.