This passive amplifier horn will amplify your phone's speaker without power. The horn profile is based on a Western Electric exponential horn. The horn is 7.5" long and press fits into the base. I added 0.010 clearance for the phone but when it printed it was still a tight fit.
Update Jan 5, 2016: I've modified the base to be friendly to printing without supports. There is still a round tube inside that may give problems without support. This depends on your printer's ability to print overhang layers. I've also increased the clearance for the phone so it's not so tight. The new base file name is signified by REV 2.
I printed this on my xyzPrinting DaVinci 1.0 with xyz filament. Used supports only on the base part and not the horn. To remove the support from inside the tube on the base I used a hair pin to push it out and it came out clean.