Your billing add-on is irrelevant for the purposes of enabling roaming, its sole purpose is to reduce costs.
What is your basis for saying "roaming services turned on" - where and how and have you done this? Many people think the "data roaming" option in their phone is this, where that is just a setting for "can I use mobile data whilst roaming" - there isn't an option for "text message roaming" or "voice call roaming", but they would be the equivalent.
Roaming is a network-level setting usually enabled by texting ROAMING to 150 before leaving the UK - did you do this?
