The simplest way to configure the OBi would be an InboundCallRoute rule on the service where the inbound call originates. This rule would route the inbound call to the Auto Attendant where the caller could place an outbound call. If the inbound caller is not verified by his CallerID, you probably should enable PIN protection on the Auto Attendant to prevent unauthorized callers from making outbound calls at your expense.
Some InboundCallRoute rule examples:
{aa} : sends all inbound callers to the Auto Attendant
{12341234567:aa},{ph} : sends CallerID 12341234567 to the Auto Attendant, all others to the PHONE Port
{12341234567:aa},{sp2(18005551212)} : sends CallerID 12341234567 to the Auto Attendant, all others to 18005551212 via SP2