Just a heads up, it appears O2 are having some issues at the moment, so if you are on O2, or trying to call someone on O2 and are having issues, this may be why.
http://status.o2.co.uk/
http://status.o2.co.uk/
Lol. Always seems to be o2 having network problems. Glad i left them 2.5 years ago.
Having issues on GiffGaff here.

Same problem (giffgaff), was with orange for nearly 2 years before not had any problems, been with giffgaff 6 months and had two outages. To anyone whos been on giffgaff longer than me: is it normally this bad?