Customer Service is terrible as my mother found out. She got an Android phone and it kept freezing when the screen went to sleep. She called them and pretty much was told that if they took it in and there was no physical fault, she'd be charged.
Now I have the same phone on o2 and I've never had an issue, even since I put the "unofficial" Gingerbread update on (Android were gonna release it but decided not to) so I figured it was the software.
After being fobbed off twice by Tesco, I flashed the phone with the Gingerbread update I have on my phone and it's worked fine every since. Clearly whatever faffing about Tesco did to brand the phone wasn't tested properly and wasn't compatible with either the phone or the original firmware.