Well, no easy answer TBH. We use google analytics and webtrends to track volumes down any single track and clicks/views but deflection is more tricky.
You could look at it from the point of view that if 100 customers hit your contact us and only 50 of them then go one to contact you from the final leg of your page then you could argue you have 50% deflection however do you really....
If you have an authenticated area such as a self help portal you could with cookie logic track a user across your online estate and through that understand where they went and if they contacted you but thats never going to be for 100% of your traffic but is more acurate than the above assumption.
Hope this helps.