Dutch or Flemish is not so different to German and vice versa.
If both parties are speaking slowly, you are able to understand it.
Yes, both languages are very close to one another and have a common ancestor-language. Dutch and Flemish - which is Dutch but spoken in the nothern part of Belgium (Flanders) - originate in medieval Lower-German ("Nederduits" or "Nederdiets")... Or at least something close to my explanation, I'm not specialised in linguistics...
To complicate things a bit for people not familiar with our languages:
"German" translates as "Duits" in Dutch, and as" Deutsch" in German, and the word "Dutch" is very close in pronounciation to "Deutsch", but "Dutch" is not the same as "Deutsch"...

But I think we're way off-topic now...
