Assuming that you are using data via the mobile network and not WiFi, then the previous post is not entirely accurate - roaming is already enabled and that's not the issue.
There may be a couple of things that could cause these symptoms. Check whether you can call local, in-country numbers or not - this is a good test for whether the problem is an international call bar.
Try swapping between local networks manually, this rules out a local network issue. This is one you can solve yourself. If the problem is an international call bar, then only EE-CS can resolve that, as per the most recent post. You'll need to make contact via an alternative means, or use live-chat via the myEE app.
