Regardless of any billing add-on, your ability to get coverage abroad (which calls, texts & mobile data rely on) is dependent on roaming being activated - which allows your phone to connect to foreign networks.
On pay-monthly, this needs to be enabled manually - usually by texting ROAMING to 150 before leaving the UK.
If you now find yourself abroad without having enabled roaming, your only recourse will be to contact CS to request it activated. You can either contact +44 7953 966150 from another working phone, or +44 800 0798586 using a web-calling app such as Skype over WiFi.
