Most GSM modems will have their own library. You do not need a library to be written in C #. Usually it will be in dll or exe. If it's a dll written using C ++, you can still call it via p / invoke. You just need to know the function signature in order to properly send and retrieve data from it. Check the documentation. If this exe is most likely a command line, then you can call it with Process.Start () and insert what you want to send through the argument.