To add, the symptoms you describe are indicative of your phone not having a connection to a local network. Whichever billing add-on you've chosen, it's separate and independent of the requirement to enable roaming on your phone - normally done by texting ROAMING to 150 before leaving the UK.
If roaming is enabled, your phone can connect to foreign networks - the ability to make/receive calls, send/receive text messages and use mobile data, naturally follows.
The opposite scenario creates the symptoms you describe, contacting CS to enable roaming - is the remedy. Some users have been able to use a web-calling app over a WiFi-connection to resolve this.
