When EU roaming, the phone will pick up the first available network that Smarty has an arrangement with. You sometimes find that it will move to a different network as you travel in a country.
As long as you are definitely within EU, then it will be fine.
If you are near a border with a non EU country, you would need to check what network you were connecting to to avoid non EU charges ( phone won’t work if you have no extra account credit anyway).
Additionally, you can manually choose a network in your Settings on the phone. The default is usually‘automatic’
