Your question isn't very descriptive, so I'm going on assumptions. If you have more information, let me know.
 
I've never used it with .NET applications, but the Java Native Interface (JNI) allows the execution of Java code from within native compiled code (such as C, or C++). You could probably make calls to the JTAPI methods through jtapi.jar, but you would probably have to write some kind of skeleton wrapper yourself, and use JNI to hit those methods.
 
Using JNI is not recommended for various reasons, but in general JNI is clunky and error-prone. If you mess up your JNI calls, you can seriously damage the JVM and cause a crash.
 
If you are looking to develop C or C++ applications that interact with the Cisco Unified Communications Manager, then I would recommend looking into Cisco TAPI. It's essentially the C++ version of JTAPI. Although there are some fundamental differences, you can accomplish pretty much anything you want with either API.

Yes, TAPI is a Microsoft spec, but Cisco has their own implementation, which has a lot more functionality, and a lot of Cisco-specific code.
 
JTAPI and TAPI are developed closely together, and while they are different in many ways, they generally have the same feature set.