I looked at OOMA, but passed on it. I switched to VOIP.MS, a great deal in my opinion. If you just want a number they charge about $1/mo for service and $0.0125 per minute for use. They charge a bit more to port your existing phone number, but my bill is still well under $5/mo.
They have a number of locations around the country/world so you can use the closest one for best latency. I have outstanding quality, and total control over my phone. They do not lock down the settings like virtually all the other providers do. I can change my rings, set when the phone will ring, I hate calls before 10AM and those that come in before that go directly to voice mail unless it's from a select list I allow. I can set up specific rings for family and route number unavailable calls directly to VM. Overall it is very powerful.