Internal Modems

100% of customers that come through my door with modem problems, all have internal modems. I’ve been operating since 2000 and not one modem problem has been external in this time, I’m sure they do fail. Can someone tell me for example why does any internal modem produce a connection problem when it passes Diagnostics from within the OS? Is it just straight hardware failure that diagnostics don?t check? Is there such a program or a procedure that can check these problems that is better than the OS diagnostics which clearly could do a better job?