If you are faithful to your husband how do you convince him of this when he is sure that you are cheating?

It maybe just his way of diverting the attention off of him, and you may want to have him checked out. That is usually behavior of someone that is not doing right themselves.

You can start convincing him by reassuring him of things he may suspect of you, i.e. always receiving blocked phone calls or talking on the phone more often than you used to. Obviously there are telltale signs that he believes you are cheating, and it is not necessarily that he is doing it himself. Some guys need the same reassurance that you love them. He may fear that other guys find you attractive and it may not help that you unknowingly respond to these guys, which some guys take as flirtation. What would you want him to do to prove he isn't cheating if the roles were reversed?

What if you have done everything to reassure your spouse that nothing is going on? Things like writing down the milage on your car, screening phone calls and not giving out phone numbers. He has open access to my cell phone and email and I have nothing to hide. But he says there is something in his head telling him that I am cheating and he can't get rid of it, nothing I do seems to convince him.

You can not. Just leave him and start to live without being big-brothered.

then he is a paranoid dillisional and very insecure...leave him and find someone that is secure!!!!