For auto and cell phone rentals in Europe we recommend Auto Europe. We have been using them ourselves for years for our staff´s inspection trips and have been very satisfied with the service and rates. We have found their staff in each of the airports in Europe quite helpful, especially for people who are new to driving in Europe.
For your convenience we have added a link below to our partnership site with Auto Europe. From there you can arrange your auto or cell phone rental.
Have a great trip!