I was in exactly the same situation and honestly didn’t expect fax apps to be this convenient. After trying a few random apps, I realized that reliability really depends on which one you choose. From my personal experience, the best fax apps let you scan documents clearly, send PDFs without issues, and give you delivery confirmation. What really helped me decide was this comparison guide:
https://geekflare.com/software/best-fax-iphone-apps/ . It reviews the best fax apps for iPhones in 2025 and explains their pros and cons in detail. I followed the recommendations there and haven’t had a single failed fax since. Now I send contracts and signed documents directly from my iPhone, even while traveling, without any stress or extra equipment.