I am using Galaxy s2 since 5 months. I have started facing Network problems recently. The mobile network goes off often and it resumes most times by itself.. but sometimes I need to restart my phone to bring it back in the network. I see the Unavailable (circle with a slash) symbol in place of the mobile signal symbol.. Also I see "SIM Refreshing data" often, not sure if it relates to the network going off.. Similar issues anyone and what did you do to fix this..
I also have a SGS2, and have had the same problem 2-3 times only. I turn on flight mode, and back. This solves the problem for me since it doesnt happen so ofthen.
if you still have the problem after this, I would re install Android to the newest version. Either do it yourself, or go to a phone shop and pay 10$ or so for them to reinstall your phone.
If this solves your problem, it was a cheap fix. If it didn't work, you can go back to the phone shop where you bought the phone and make them fix it in guarantee.
If you want to try to download and install an awesome rom for your phone, you should read this: http://www.cyanogenmod.com/devices
I also have a Samsung Galaxy S2, and experienced the same issue before.
It's possible that the ROM or the radio (firmware) has problems. If so, you should try to update them.
I am now using a custom ROM (CyanogenMod, latest nightly), and have not encountered this issue so far.