Cell Phone Carriers in Frankfrut?

Trying to be proactive on an upcoming Germany trip: How can I find out prior to my arrival who are the local cell phone providers in the Frankfrut, Germany area?

There are four mobile network operators in Germany: T-Mobile, Vodafone, E-Plus and O2.

Unless you’re doing something fancy with data, or buy a local SIM, does it matter? The way it works normally is, if you go to a foreign country, your phone selects the most appropriate network for you at any given time. Usually the one with the strongest signal. When I’m abroad I quite often find that I change providers several times a day as I walk around - but, and this is crucial, you pay the same rate, because you pay your home network, which has reciprocal deals with all providers.