I have Passport and use when I am in Spain. I found that sometimes I was being charged the Passport Rate and sometimes not. When I queried this, they told me the way to ensure this does not happen is to switch your phone to Manual Network, NOT automatic. If you are on automatic, you can check that your phone is on the correct tarrif (all seems OK) and then it roams to another. If you do it manually, this will not happen.
Wanted to use it when I went to Japan, but my Nokia wouldn’t work there. Apparently Most phones won’t work there as it is a different system.