If you can receive incoming calls to your account DID your registration is OK and the adapter responds correctly to the incoming call signalling which probably means that you have the account credentials configured correctly.
If you pickup the phone and get a fast busy immediately before dialing, that's one thing. If the busy is a fast busy and it comes either while you are dialing the number or immediately after then that often points to a dial plan problem or it could be an immediate rejection by your voip provider validating your account or an immediate rejection of the number you have dialed. If the busy comes after a few seconds then that points to a problem your voip provider has terminating the call.
You should first try to determine if the call is actually sent to the voip provider and the busy signal is due to some problem there, or does the OBi reject the call before it is sent due to some dial plan or other configuration problem. If the call doesn't make it to the OBi Call History that you can access under the Status Tab then the call probably was not actually sent to the voip provider. If the call is found in the Obi Call History then there will probably be some error code or message that is returned by your voip provider that caused the busy signal.