My wife's Tesco mobile packed in so she took it to tesco service. After swapping SIMS and performing various tricks she was told it was faulty and purchased a new phone €160.
A couple of weeks later my son had a look at it and fixed it. It turned out it was a dual SIM phone and it was set to the wrong SIM, I don't know how it changed but it doesn't say much for Tesco service guy, and they managed to get an unnecessary sale from use