If the iPhone will work depends on the version of the iPhone. Is it an very old iPhone 4 or is it an 4S, 5, 5C or 5S? The very old Verizon iPhone 4 will not work!! The first generation of Verizon iPhones did not support the GSM/UMTS network. So they don't work in Germany!

With the newer ones you will have no problem. They support the 2100Ghz UMTS frequency used by all carriers in Germany / Europe. If you have a 4G/LTE iPhone it supports the LTE frequency of 1800Ghz (band 3). So you will have no problems.

Just buy a prepaid sim-card. The cards will have voice and data. Depending of you want to do you might want to to go for one that has more data than voice or one that offers cheap international calling (for calling home to the US). The best networks are provided by Deutsche Telekom and Vodafone but are a bit pricier. Vodafone has a lot of MVNOs that sell under their own brand but use the Vodafone network. The two other carriers o2 and E-Plus have a bit weaker net but are cheaper. The cheapest MNVOs normally use the E-Plus or o2 network.

http://www.german-way.com/travel-and...ne-in-germany/