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.
If you are on pay-monthly and 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 9566000 using a web-calling app such as Skype over WiFi.
