Once you reach the OBi Auto Attendant (AA), you can press 1 to "continue this call", i.e. to ring the OBi phone (FXS).
Or press 2 to "make a new call", i.e. dial a new number, let's say 1-800-xxx-xxxx (US number) or 011-44-xxxx .. (int'l call to UK). Make sure to press the # key at the end of the dialing.
You can always "barge-in" your dialing of 1 or 2, the AA menu will comply accordingly.
For an expert user, this AA can be added to your "Inbound CallRoute" parameter, e.g. {aa, ph}
For ease of use, go signup at ObiTALK page, so you can manage your own device easily, especially to invoke the AA and other things.