What to do when professor is not responding to emails or calls and his action is required to commence a masters thesis? I'm planning to start my master thesis soon. It will be an external thesis in a company.
As this kind of thesis is not very common in German universities, I need to find a professor to supervise me during the whole semester.  

Luckily, a professor agreed to supervise me and advised me to meet with one of his PhD students to discuss my task, whether it meets all requirements.
We met the following day. Afterwards, I sent my professor an email to arrange a meeting to clarify some things and get his final OK.
As he hadn't responded for about a week, I sent a reminder. This time he answered, we met and discussed more things. He even suggested to call my boss to clarify my task, so that I don't need to "liaise" between them.
After talking to my boss, I sent my professor another email with times, my boss is available for the call.
This email remained unanswered for another week, as well. I wasn't sure how to react, so I contacted my professor's secretary. She told me to send him another reminder, also she would talk to him.
He answered me a few days later and promised, to call my boss during the following 2 days.
Unfortunately, he didn't.

I'm in an awkward situation now. My boss asks me regularly about my professor, because I can't officially start my thesis before we have his OK. I'm waiting for about a month now.
The phone call between them wouldn't last more than 10 minutes and all the little issues about my thesis would be resolved.
His secretary said, this behaviour is very uncommon. Also, I never had any problems with this particular professor.
You didn't really ask a clear question but I will answer the implied question: "How would you handle this situation?"

My experience with emails and professors is they are hit and miss. For some this is a very reliable method of communication, for others it is not. This isn't necessarily generational since quite often universities give young professors a new email address which becomes just one more thing they must monitor. In addition, as the semester progresses, most professors' inboxes become flooded with student email along with the usual administrative junk mail.   

My suggestion would be find some way, phone call, in person visit or "ambush" outside of a regularly scheduled class. Once you find a way to make contact, explain your predicament with your boss and ask if there is a better method of communication. If the reply is "just use email" then explain this has proven unreliable and if you are to continue with the collaboration you need another method.  

Bottom line is this is a collaboration.  If you are unable to collaborate, you will not finish your research and you will damage your reputation at your job.  If the professor is unable to communicate in a manner supporting the collaboration, you need to find a different professor of find a different thesis.