Look at your appliances FIRST. If your computer, for example, can run off of 110-240V (it will say that somewhere, usually on the bottom), you don't need a converter, just a plug adapter. My grungy old laptop worked everywhere in Europe, and all I needed for it was a standard European plug adapter. European plugs--with the exception of Britain and Ireland--are round, two-pronged plugs that sometimes have a hole for ground (older European wall outlets sometimes have a ground sticking out of the wall that fits into a corresponding hole on the plug, in an odd reversal from what we're used to in the U.S.) If your computer will NOT run on dual-voltage, then yes... you'll need a converter. Converters are generally expensive and heavy, so be sure you really need one before you get one. I'd also recommending getting a few American-to-European plug adapters, instead of just one, if possible. Most travel stores and even airports will sell them; I find that they're sometimes cheaper in Europe, actually, but it's up to you where and when you buy them.
Now, as for the phone; the phone will work 1.) if it's GSM and has a sim card, and 2.) if your phone is tri-band at least, if not quad-band. GSM phones in the US (and not all phones in the US are GSM/sim card anymore) operate at the 1900 mHz frequency; whereas European countries are on either 800mHz or 1800mHz (unless those frequencies have changed since I last looked). That's where the term "tri-band" comes from. Check and see if your wall adapter for your phone is dual-voltage (if it says 110-240V or 110-220V). If so, you only need a wall adapter, not a converter, as I said before. If not, see if the manufacturer has a wall adapter for your phone model that IS dual-voltage. Also, bear in mind that you can go to any electonics store in Germany, like MediaMarkt, and just buy a cheap prepaid cellphone (a cellphone is called a "handy" in Germany) and buy 10 Euro, 20 Euro, 30 Euro, etc. phone cards for it. T-Mobile (D1), Vodafone, and O2 are the primary carriers in Germany. Bear in mind that cell phone plans (prepaid or normal) in Germany or anywhere else in Europe tend to be much more expensive than in the U.S.; and that all incoming calls are free, and calls to other cellphones are cheaper than calls to any landline.
If you need to know more, I'd be happy to answer further within the realm of my own experience.