Norwegian pre-paid GSM in German Phone

[JonOhioValley]

We have a basic pre-paid phone we purchased in Germany last year. We are returning later this month and will use the phone in Germany. We also will use the phone first in Norway, then Denmark and then Germany. We may also need the phone in Sweden, Belgium, Portugal, Spain and Italy. Should we purchase pre-paid GSM cards in several of the other countries or just add minutes on the German card? What is involved with purchasing a pre-paid GSM card in a country different from where we purchased the phone; how is the new card activated, etc? Thanks for any help