Stephan,

I do not believe that Avast would detect this dialer.
Dialers are more trojan than virus.

Spybot most certainly detects dialers, but this one may not be on the list. You can add it.

Adaware is also very effective.

I would send an email to support for Adaware at Lavasoft and to Spybot. They should then add the file to their signature list.

A trojan detection program called A2 by Andreas Haak will detect this dialer. The program is freeware as far as I know but has some shortcomings.

Good luck