I found a service where you can have a UK telephone number assigned to you (for free) which can be redirected to phone numbers in a few different countries, including Germany. The 0844 numbers cost 5 pence per minute to UK callers (from a BT line).
http://www.esms.com/services_numbers_pure_free.php
It works fine for me - there area few squeaks on the line during calls, so I guess it uses VOIP.