.... is cost the be all and end all.
I could easily go for a Tesco phone here - but signal would be zero, so while my mobile would cost less to run, I'd be pumping a lot of 60ps into call boxes. Same for anyone piggybacking on O2 in fact.
I have now had a phone with each of the networks and 2 supermarkets. Leaving aside coverage issues none of them are customer-focused and none communicate well - they're all universally crap at it - which is simultaneously galling and amusing when they're trying to be communications providers.
Which? I prefer Andrex - it's more absorbent (and a better read).