When you receive an inbound call the service provider passes 2 fields to the OBi. The 1st is the CallerID and the 2nd is usually your username of the trunk. When you bridge a call and just specify a destination, the OBi will use the username as the outbound number. You need to use a null field to get dial tone on a PSTN.
Try this: li('')
That's 2 single quotes.