When you pick up the phone and find out that it is a call from the bank,
never agree to receive information by phone. Do not answer any
questions, even if they seem harmless and simple.
Take the initiative. Clearly and with no compromise, very politely insist on a communication channel via email. If they say per email would be not possible, suggest the letter via postal way. They cannot reject the postal way because it prescribed by German law.
You do not need to explain this wish. Say that you feel sorry and thank repeatedly for the understanding and consideration. Why? Because the organisations and companies don't care about your interests. They always follow their interests.
Example: Imagine the insurance company employee or an official from let's say immigration authority is calling you to share important information. For example, the information that will reduce your insurance rates and may save you some money or that three documents are missing and they cannot go on the process of issuing a permanent residence permit card.
After this call that peron will make a note in the files that "you were contacted and informed about such and such".
What can happen when you receive important information on your site by phone?
Mind the advantages which an email offers compared to phone calls (and personal visit!):
In a personal visit I would not judge as categorically as in the previous telephone vs. mail comparison.
Actually you don't have that much choice if a personal visit is simply necessary. You just have to go. Here we talk about situations when you have the choice - go there in person or clarify the issue by phone or in writing.
Most of the downsides mentioned above (insufficient or misunderstood communicative interactions etc.) apply to face-to-face communication as well as on the communication via phone. Body language is not a good help when it comes to paragraphs and written texts. Besides, the visits to authorities cost an incredible amount of time. Your time.
The positive thing about a personal visit is the opportunity to establish a good personal contact with the employee/official. Surely, there is no guarantee that personal contact and a friendly relationship will occure. However it is always worth trying. Because an official who find you charmant is a great allied person that can do a lot of things for you that are considered as "absolutely impossible".
An official in Germany can sometimes be indeed very cooperative when
suddently a sympathy and "good chemistry" on both sides occures.
This phenomenon even has a name. In the world of government officials
it is called "nach Ermessen", in commercial structures it is known as "Kulanz".
More about this here and here.
However good and friendly relationships with officials are rather the exception. People just pretend to be objective and treat all the applicants equally. (Whch I personally do not believe.) So if there is a friendly relationship with any decision maker, my recommendation is to care about it well.
You have received information or answered questions by phone, which you did not quite understand or simply misunderstood. What would be the worst case?
It is most unlikely that there are serious consequences (such as deportation or criminal records). Most likely in the worst case, you will probably pay a few hundred or thousand Euros and the issue is solved.
So ask yourself whether a misunderstanding has to cost you that much?