It is a problem. The other advice is to make sure your phone has a security lock ( password etc to use the phone). I know these can be easily hacked but it can give you some time to call your service provider. when my phone was taken first thing I did was call EE and tell them my phone was just stolen. First thing they did was lock the phone completely. I was told it meant that even if they managed to open my screen lock that nothing would work. From what I remember they said it basically wipes the [hone and kills it. Not sure it that's true but I never had an issue after the phone was gone. So keep the customer service number of your cell provider and call them as soon as possible. You can also try the find my phone or some such thing. If you have a google or apple account that has credit/debit cards attached make sure and have them disassociated from that account.

Don't keep your phone in a back pocket or hoodie pocket without having your hand on it. This is true on busses and the tube.