Overview
Microsoft offers several forms of technical support: standard telephone support, priority (fee-based) telephone support, and a searchable knowledge base. To find out more, select Help - About Microsoft Excel, then click the Tech Support button.
Before you spend any money, make sure that you've exhausted the other resources described here. Then pay a visit to one or all of these sites:
Use the phone?
My experience is that you should use Microsoft's telephone support only as a last resort. Chances are, you'll run up a big bill (assuming you can even get through), spend lots of time on hold, and may or may not find an answer to your question.
The truth is, the people who answer the phone are equipped to answer only the most basic questions. And the answers to these basic questions are usually readily available elsewhere.