Per fax.plus:
Data Encryption In Transit and At Rest
Fax files at rest are encrypted using 256-bit Advanced Encryption Standard (AES). To protect data in transit between FAX.PLUS apps (currently mobile, API, or web) and our servers, FAX.PLUS uses Secure Sockets Layer (SSL)/Transport Layer Security (TLS) for data transfer, creating a secure tunnel protected by 128-bit or higher Advanced Encryption Standard (AES) encryption.
I don't know if that's good enough but I'm guessing it's as safe as a physical fax line and documents sitting in the fax machine overnight. They are HIPPA compliant, is that good enough?