If you have installed version 7 of Azure MFA Server on-premises (18.104.22.168 or 22.214.171.124 at the time of writing) and have enabled IIS authentication with Forms Based authentication and the Native App, but when you need to authenticate you are presented with a 503 DLL error. The reason for this is that version 7 removed support for 32 bit Windows, but if the application pool in IIS for the website you are running is a 32 bit pool then the 64 bit DLL provided by MFA for authentication will not run. If you change the pool to 64 bit then the MFA authentication DLL will work, and your phone call/text or mobile app verification should occur. Of course, if you change the application pool to 64bit make sure that other DLLs used by the application are not 32 bit and so the application itself, rather than MFA, would not fail.
If the application is 32 bit and therefore the application pool needs to be using 32bit MFA DLL’s then you either need to upgrade your application to 64 bit or downgrade MFA Server to version 6.3. To obtain version 6.3 you need to raise a support call with Microsoft.