I got the following code to create a button on my webpage to start a Skypecall:
I tried some stuff. The best way would be what Joe Clay commented above. But I still got the problem that I couldn't make calls to classical phone numbers like
After some research I found out that I simply could use
This makes Skype4Business to ask if you want to call the given number. Notice that Skype for Business must be your default application to deal with
tel: links. You can change that by searching for "default programs" on your computer.
I hope this also helps some other people.