Hi Rüdiger,
As far as I know, accepting the call (by VoIP, land line, or cell phone) shouldn't pose any threat.
What they want you to do is to allow them to remotely login to your PC and "fix" your machine for you. That is so obviously a security threat that I'm surprised anyone falls for it, but I'm sure enough people do that it pays them to call me at least once a month hoping I'll fall for it that day. (Even though I don't have any systems in my house running a Windows OS, and it is impossible to follow their directions to give them remote access.)
Cheers,
Don