You may keep your same number by porting it to the VoIP service. First, you'll need to see if porting your current number to a new service will affect your existing service(s). In most cases, your Internet will continue to be served by the same provider, but with OBi and a VoIP service, you will no longer need to maintain the phone service from this same provider. Hence, you will need to check with your current provider if it is feasible to stop using them for voice service, but keep their Internet service.
Although your specific experience may vary, faxing usually works great as long as the Internet connection is good. Longer faxes (+10 pages) tend to have the most problems due to inherent issues with faxing over the Internet.