This thread is so big many points get lost. Here is a point that everyone travelling to Europe should understand - FOR EUROPE, YOU GET A MUCH BETTER DEAL BUYING AN ESIM OFERED BY LOCAL CARRIERS THAN YOU GET WITH SAILY, AIRALO, AND THE OTHER DATA BROKERS.

For example, you can easily buy an Orange SIM here: https://www.esimholidays.com/en for 10 euro that gives you 80 GB in Spain and 15 GB in the rest of Europe. Because you are getting a real local SIM it has a phone number and 500 minutes in calls included. 5 Euro more gets you 260 GB in Spain, 22 GB in the rest of Europe.

For a recent trip to Spain, I bought an eSIM from vodafone.es for similar pricing and data allowance, which also included US data roaming.

I recently bought an eSIM from Lycamobile UK for £5.00 that included 30 GB of UK data, 12 GB of EU data, and 100 international calls.

These are just examples. In all cases I downloaded the eSIM before I left the US and it worked upon arrival.

The Orange SIM worked in Europe without first activating in Spain - in fact, I never used that SIM in Spain.. The one caveat is not to buy a local carrier SIM before leaving the US unless your trip takes you to the country of the SIM first or you have confirmed that the SIM need not be activated first in the SIM's home country.