I've been using an iPhone 4 on a Vodafone contract for the past 16 month and just bought an iPhone 4S direct from Apple. It arrived on Friday morning and I got it set up correctly as far as I can tell.
When I first turn it on, it works lovely and exactly as I would expect, able to make calls, send texts and access the internet via the cellular network. However, at what appears to be random, the network will disappear (no bars of signal) stopping me making calls and texts, but still allowing me to access the internet. Sometimes, the Internet goes, but I can still make calls and texts. Other times, everything is gone and I get "No Signal" where it normally says Vodafone UK.
So when trying to fix this problem after it's noticed, I try putting it into Airplane mode and back out. The phone scans for a network and goes back to the same state as before I tried this fix. I have tried resetting network settings, but this doesn't change anything. The only thing that actually fixes it temporarily is turning the phone right off and back on again.
At this stage I assume this is a bug with the software, but I wondered if anyone else is experiencing anything like this? If no one is, I will take it up with Apple, but I don't really want to do that as I love the new phone so much!!
Same issue here in Germany with T-Mobile.
The only way to overcome this is to disable the SIM PIN.
Not a real solution but that solves the issues for now.