But is it possible to compile this dll into Qt?
It is important to understand:C++ is a language. PJSIP is a C++ library. Qt is a C++ library (mostly). Qt Creator is an IDE. Visual Studio is an IDE. Microsoft Visual C++ is a compiler.
You use an IDE to write code and build applications/libraries. You need to tell your IDE which compiler to use.
You can use Visual Studio to write C++ applications that link to the Qt and PJSIP libraries.
You can use Qt Creator to write C++ applications that link to the Qt and PJSIP libraries.
How can I build a Dll in MSVC2019 (I created a library project)?
You must follow the instructions from PJSIP documentation. We do not know how to build PJSIP.