This is just a note for posterity in case someone else hits the same issue. My naive attempts to search for the error displayed did not yield any results, hence this post.
I arrived in the US yesterday and signed up for the 7-Day Zone 1 Pass on arrival on an Android phone (Pixel 7).
When trying to make calls a little while later, calls would fail and automatically hang up. There was brief flash of the message "server not available". Only one telephone number I tried worked (Bank of America), the other half dozen all failed with the "server not available" message.
Data appeared to be working okay though.
The fix was to go to the EE SIM settings and turn off "automatically select network" and then manually setting the network with the "Choose network" option.
What I think happened is that the phone attached to the AT&T network when I arrived and setup the pass. Later on, the phone appears to have automatically switched to T-Mobile which didn't want to support voice calls. Manually selecting AT&T fixed the problem.
