I had the same concern when a government office specifically asked for documents “by fax only.” After researching and testing it myself, I found that mobile faxing is legally accepted in most cases because it follows the same transmission standards. What helped clarify this for me was https://www.techspot.com/news/106844-how-fax-phone-secure-reliable-effortless.html , which explains how phone-based fax services work behind the scenes. From my experience, as long as the fax is delivered successfully and you receive a confirmation report, it’s treated exactly like a traditional fax. My advice: always keep your delivery confirmation and timestamps. They act as proof and can be extremely useful if any disputes arise.