I used to think that too until last week. I have a sony ericsson T300, and one of my co-workers has a Motorola V600. It was during the night shift, and him and myself were taking turn so one would stay on post (security guard) in the lobby, while the other would be underground sleeping. When I was underground, he couldn't contact me, but when he was underground, he still was getting every single calls made to his phone. That's how I realized that not only the service provider matter when it comes to signal, but also having a phone with good reception.