Recently despite having 2 contracts with 3UK I'm using my Giffgaff sim more and more.
The reason why?
My other phone being on 3 it's either 3g or nothing. Giffgaff provides either 2g or 3g support.
I've missed important calls on my 3UK phone because for some reason dispite having a full 3g coverage my phone diverts to voicemail without even ringing. And i've checked the settings and all seem to be in order.
My Giffgaff sim JUST WORKS it's being put into a 2g handset and i've never missed a call.
Why? because it uses O2 as its parent company and i believe O2 is the best at what it does.