QB Issue Resolution:

You can add the fax number in a custom box to the sales form (usually appears at the bottom of the form)

Resolution for Issue 'How to add fax number to sales invoice?' available: Yes (Solved).
Source: Intuit Community forum.

How to add fax number to sales invoice?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Customize Invoice
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Error Recovery for Customize Invoice [Canada]
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