You can define the call direction rely on an event with the extension id in the parties object. See this example.

// No extensionId => internal call

"parties": [

        {

          "accountId": "809646016",

          "id": "p-a0d7bca1f4a83z18613348f51z3343dxxx",

          "direction": "Outbound",

          "to": {

...

// has extensionId => call to/from this extension

"parties": [

        {

          "accountId": "809646XXX",

          "extensionId": "62242421XXX",

          "id": "p-a0d7bca1f4a83z18613348f51z3343dxxx",

          "direction": "Inbound",

          "to": {

You can read this blog for more explanation about telephony session events.