In 20+ years of using mobile phones on different networks the only times I've had to phone a customer service call centre were usually to request a PAC when leaving a network. This is something that can be done very easily online with giffgaff.
I also had to phone Vodafone's call centre numerous times after they'd yet again cocked something up on my account. This is not something I've ever experienced on giffgaff.
I once called Orange's technical support to find out the correct APN settings for tethering my laptop to my phone. The settings they gave me were incorrect and didn't work. In the end I just googled for the correct settings and found them quite easily.
Based on the above I'm really wonder why some people are so attached to call centres! I quite happily wouldn't ever phone a call centre again if I could avoid it.