This all comes down to who your service provider is. They may require different formats. Generally in the us, 2223334444 would work. Some may require 12223334444.
Depending upon your provider, you may need top prefix this with a command to change the codec to and/or to set the fax machine with a slow speed and perhaps turn off error correction.
If you just bought an Obi and plugged it into a router, you have more setup to do.
Which Obi did you buy?
Are you using GoogleVoice,
Voip.Ms? Callcentric? Voipo or someone else as a provider?