@iTraxx Welcome to the community and thanks for posting, I'm really sorry about the problems with your BT Mobile service.
Our Mobile team would like to get this investigated further and have requested your customer details to help them resolve this problem and I've sent you a Private message explaining how you can contact the forum moderation team.
Thanks you very much for bringing this to our attention.
Our technical team has investigated and have now confirmed this being caused by a technical issue with one of our systems. Incoming calls while in Roam like Home countries are free to receive, so it shouldn’t matter that the spend cap has been set to zero (£0), but the system isn’t recognising this. We are working on a fix for this issue, but It may take some time for us to rectify the situation, but we’ll let you know as soon as it’s fixed.
In the meantime, please do be assured that you aren’t being charged for these calls, and the workaround that you have already found , i.e, setting the spend cap to £2.50 will do the trick until such time we implement a fix.
We’re sorry for the confusion around this issue, and thank you again for highlighting it and for your patience while we investigated.