Speaking as a one-time 'high speed internet tech support customer relations specialist' working at a call-center contracted out to Comcast, I was told that it was close to 90%. That seems exaggerated since I would say it was closer to 70% (ballpark) success for my calls, but it was high enough that the place eventually implement an entire first-line crew that did no more than ask about whether power cycling had been complete or explain power-cycling before handing the 'sub(scriber)' off.

If someone called in with the GP's depth of troubleshooting completed, the standard policy was say something like 'this is outside of my allowable offered support'(but if you really know how, help, but not officially!).

I'm usually hoping that they'll send me on to level 2 support so I don't have to redo all of the stuff I've already tried. I won't even try calling support until I've exhausted their FAQs/Knowledgebase and Stackexchange/Google. Picking up the phone is the last resort. Unfortunately it's fairly rare that the phone support is of any help.