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 PAYG, this is auto-enabled once your phone is used in the UK first.
On pay-monthly, this needs to be enabled manually - usually by texting ROAMING to 150 before leaving the UK.
If you are on PAYG and now find yourself abroad without having enabled roaming, the only recourse is to use that SIM on EE's UK network first. There is no workaround.
