Hello, i joined giffgaff this week. I ported my number which was meant to switch yesterday 20.08.2015. My old number stopped working and its showing online that my number has changed over to giffgaff but my phone is showing "no service". Can someone please help me with this.
Is your phone unlocked (or at least locked to O2)? Even an unactivated giffgaff SIM will display signal bars ('dots' on an iPhone).
No signal bars is a pretty clear sign that the phone is locked to another network.
Cn you try the SIM in another (unlocked) phone?