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.
Have you used your phone abroad previously, did you receive a welcome text message on first arrival abroad? These are good evidence of roaming being enabled if so. In these cases, manually cycling through all available local networks often resolves issues - toggling flight mode can have the same effect.
If not...On PAYG, roaming is auto-enabled once your phone is used in the UK first. Did you do this?
