If you are travelling in the EU with an active Guido membership then calls/texts/data will work. You don't need a data add-on unless the Guido 1GB allowance is not enough for you. In that case add add-on will also work while roaming in the EU. If you roam in the EU on a membership other than Guido then nothing will work at all. If you are roaming elsewhere (North America, Oceania) then you need any membership at all but also cash credit to cover the costs of outgoing texts and outgoing/incoming calls. When roaming outside of the EU data will not work.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.