And the fact that you're not likely to answer a call from an international number because it'll probably look more like spam.
Also, for them to have a +1 number outside the USA means they have to be on a USA network and then roaming onto a local network. This presents dozens of problems, such as often not being able to get the best connection, not being able to get data connections, not being able to get any local support, and it costing a small fortune.
All the embassy employees I have ever known have gone full native with all of their technology etc.