[DllImport("cellcore.dll")] private static extern int lineSendUSSD( IntPtr hLine, byte[] lpbUSSD, int dwUSSDSize, int dwFlags);
Tapi tapi = new Tapi(); tapi.Initialize(); IntPtr ptrHLine = tapi.CreateLine(/*...*/); byte[] dlgArr = Unicode.GetBytes(@"##21\0#"); lineSendUSSD(ptrHLine,dlgArr, dlgArr.Length, 0);