1. Check the signal strength on the phone screen, and move to an area with better reception. 
  2. Remove any accessory covers or shells from the phone temporarily to check if they affect the signal strength.
  3. Check to see if the screen stays active while in a call. If you have a touch screen phone, you may accidentally press the hold call or end call button on the screen with your cheek.
  4. Check that your SIM card is fully inserted. Do not use SIM cards that have been modified to fit the SIM slot.
  5. If possible, try your SIM card in another phone.

If you continuously experience low signal strength in a certain area, network coverage may be insufficient there. For more info, contact your network operator.