Welcome to the UK firstly.
What phone do you have? Also is your phone on manual or automatic roaming?
If your not sure how do that post the model number and brand and im sure we can help you check.
The wife has used a roaming sim and with some companies it worked others it did not.
What mrs Daddy says about minimum load i can remeber being an issue.
you can purchase load on line in the UK and im sure from some phill stores.
