Given what I am gathering from this, you are able to navigate within the phone but you are unable to make or receive calls and such, right?
What kind of phone do you have?
I noticed you tagged Android. Go into the settings and look for Mobile Networks.
Knowing the exact phone would help us provide details how to get to those settings IF you are unaware how to. If you do, Make sure that 2G is off or the Network Mode is set to GSM/WCDMA AUTO or GSM/HSPA/LTE AUTO or GSM/HSPA AUTO