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Send Faxes from Formstack Documents with Sinch

Ron S.
Updated

Integration Overview

Use this integration to automatically fax documents you generate in Formstack Documents through Sinch. When a document merges, Formstack sends the output to Sinch along with the destination fax number. Sinch delivers the fax and can optionally send a callback to your system when delivery succeeds.

What the integration does:

  • Sends merged documents from Formstack Documents to Sinch for fax delivery
     
  • Supports delivery status callbacks
     
  • Flexible delivery - delay a fax until later 
     
  • Reference the delivery status and the faxed PDF directly in your Sinch dashboard

     

Who this helps:

  • Formstack Documents customers who need a secure, reliable way to send faxes as a part of their document workflows
     
  • Teams that require delivery confirmation for compliance or audit trails (e.g., proof that a records request was successfully faxed)

     

Integration Setup 

Before you begin, you’ll need:

  • A Sinch account with access to Access Keys and a Project ID
  • A Formstack Documents document/template that merges successfully
  • A fax number that can receive faxes (for testing)
     

Step 1: Get your Sinch credentials

  1. Sign in to your Sinch (Phaxio) dashboard.

     
  2. Go to Settings ▸ Access Keys.
  3. Copy the Project ID.
     
  4. Click Create Access Key (if needed) and securely copy the Access Key ID and Secret.
     

You will use: Access Key ID, Access Key Secret, and Project ID.

 

Step 2: Add the Sinch delivery in Formstack Documents

  1. In Formstack Documents, open your document and go to Deliveries.
  2. Click New Delivery.
  3. Open the Other tab and choose Sinch.

  1. Enter the required fields:
     

    • Sinch API Key (Access Key ID)
       
    • Sinch API Secret (Access Key Secret)
       
    • Sinch Project ID
       
    • Fax Number (E.164 format, e.g., +1XXXXXXXXXX for U.S.)

    There are additional features available under the "+ More options"  button: See below.

  2. (Optional) Configure Callback Webhook URL if you want delivery status posted back to your system (e.g., “fax sent correctly”). 
  3. (Optional) Delay Delivery to send the fax later at a later date. 
  4. (Optional) Set an Error Notification Email if someone other than the account owner should be alerted if a fax delivery fails.
  5. Save the delivery.

     

Step 3: Test the integration

  1. Trigger a test merge of your document.
     
  2. In the Formstack Documents ▸ Overview tab, confirm the delivery shows as Sent to Sinch and note the Fax ID returned by Sinch.
     
  3. In your Sinch dashboard ▸ Faxes, locate the same Fax ID and watch the Status (e.g., In progress, Completed, Failed).
     
  4. Click View Fax then Download PDF to verify the merged content and any default headers/stamps applied by Sinch.

     

Check the delivery status

  • Formstack Documents: On the “Overview” panel, it shows when the merged document was sent to Sinch and the returned Fax ID.

  • Sinch: The Faxes page lists each fax with its current Status; you can open and download the PDF for confirmation/records.



 

  • Webhook (optional): If configured, your system will receive a callback when Sinch confirms receipt by the destination.

     

FAQs

  1. Do existing Phaxio users need to switch?
    Existing users can continue using their current setup. New customers will use this Sinch integration.
  2. Can I set a custom Caller ID/header?
    This option is not currently available in the Sinch delivery.
  3. Can I track fax delivery automatically?
    Yes. Configure the Callback Webhook URL to receive confirmation updates directly into your own system or CRM.
  4. Does this integration support HIPAA compliance?
    Sinch and Formstack both follow secure data handling standards. You can maintain HIPAA compliance by ensuring faxed documents do not include unnecessary PHI and your Sinch account is configured under a signed BAA.
  5. What’s the advantage of using Sinch over manual faxing?
    This integration eliminates manual sending, provides instant confirmation, and integrates faxing directly into your Formstack Documents workflows - keeping your team compliant and saving them time. 

 

Troubleshooting

  • Fax stuck In Progress: Verify the destination fax number is correct and reachable; try a different test number/app.
  • Delivery failed: Check your Sinch credentials (Key, Secret, Project ID) and the fax number format (+ plus country code). Review Sinch dashboard error details.

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