If communication switches from English to German, you should stay on a first-name basis but retain the formal Sie when you address your German partner. In Germany, people guard their privacy, so do not telephone an executive at home without permission. They also regard their professional life and achievements as more important than their privacy. That means you can telephone them at home. As well as being formal, negotiations are direct.
German managers speak their mind.
They place great weight on the clarity of the subject matter and get to the point quickly. This has advantages, especially in complex negotiations. Because German business people tend to approach negotiations systematically, communication is easy to establish and maintain. You may occasionally run across business partners who are rather dogmatic. If so, you may struggle to persuade the other side to reach compromises.