A 403 is given by some providers if the device is not registered. If you were previously using another device or app with that account, check that it has been shut down.
Otherwise, please post: Provider? Modem make/model? Separate router, if any? In the OBi System Status, what is shown for SP1 and SP2? On a failing incoming call, what does caller hear? Immediately or after a long delay? What, if anything, appears in Call History for the failing call? On a failing outgoing call, does Call History show the correct Peer Number? Any other clues about the error, besides the 403? Does your provider account work properly from a softphone or other device (shut down the OBi when you do this test)? Anything else that may be relevant, e.g. OBi working ok with another provider, other VoIP devices or apps on your LAN, etc?