Network problems with Samsung Galaxy Young (S6312)
I have had a similar problem to this. In my case, it was caused by the SIM card coming loose. Try the following steps:-


Turn the phone off completely
Remove the SIM card
Reinsert the SIM card
Turn the phone on again


and then see if you can make a call and use mobile Internet. If it happens repeatedly, you might need to wedge a small piece of paper in with your SIM card to keep it in place.