A client who called recently was puzzled about why she was seeing a strange 'phone with arrow' symbol at the top of her iPhone screen, next to her 3G/WiFi symbol. This is the 'Call Forwarding' symbol, and usually indicates that your phone has been forwarded to a different number. However, she had not activated call forwarding and was concerned that this meant that her calls weren't going to get through.
I have had a couple of clients who have seen this symbol suddenly appear. This seems to be a small glitch suffered by some devices and may be carrier related.
Don't worry if you see it. Just go to Settings -> Phone.
Touch on the Call Forwarding option. You should see that the Call Forwarding 'switch' is off. If it is green (on), slide it across to the other side to switch it off.
The 'phone' symbol should then disappear from the top of the iPhone screen. If the symbol appears next time you upgrade or restart the device, don't worry. Just follow the same procedure to get rid of the symbol.