modify your Inbound DID to route to "EXTEN" and put the toll free number in the extension field. You will likely need to add "91" or "1" to the front of the extension as if you were dialing the toll free number from a phone on the system.
An inbound call is one that a customer initiates to a call center or contact center. A help desk handles toll free number as well, although calls may be made from employees rather than customers. A call center may handle either inbound or outbound calls exclusively or might deal with a combination of the two.