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I guessed that you have a dedicated company fax number and you want to receive inbound fax notification to that particular fax number. In that case, login to your account service web, select the "Phone System" tab and under the Phone Numbers menu, select All Numbers then click on the fax number to open the details. Check how your fax number is set.

If it is set deliver to an operator, check your auto-receptionist to see which extension is set to handle faxes, authenticate your app with that user login credentials to receive notifications.

If it is set to an extension, authenticate your app with that user login credentials to receive notifications.

Unfortunately, the /fax event notification does not support phone number filter parameter. You would need to parse the to phone number (the subject) when you receive a notification in your app.