If you need to contact FedEx customer support about a package on the phone you can call the FedEx customer support number at 1.800.463.3339
If you're unable to get your tracking number from the shipper of the package FedEx customer service can sometimes still find the package that you're trying to track or solve any other issues you may have with a FedEx package.
You can also fill out a form on FedEx website and send FedEx customer support an email about your package if you prefer to email instead of talk over the phone.
To send FedEx an email go to this link
http://www.fedex.com/us/customersupport/email/index.html
Although you'll get faster customer support by calling FedEx instead.