Here's a list of things to do.
1. remove dos.h
2. remove conio.h
3. remove process.h
All of those are obsolete.

4. remove gets(), the world's most dangerous function.
It's been removed from the standard. You have no reason at all to ever use it.

5. remove all those fflush(stdin) calls.

6. remove your main: label and those backward goto's.
Use a while loop.

7. Your main is a 500 line bloat-fest, not helped by double spacing everything.
Each one of your cases should be a separate function, taking appropriate parameters.
Code:
case 1:
  add_contact();
  break;
case 2:
  list_contact();
  break;
8. Put all those global variables in main.
Then pass parameters to the functions you implemented fixing point 7.