Supernova 7210 network problem

Hello,

I am using Nokia 7210 Supernova, from past few months I am having troubles with my network, it seems that my phone is not able to catch network ( vodafone ) which I use. Its like whenever I am outside in field then I full network but as soon as I get in my room or somewhere in closed areas then I don't get it. ( all those in the room who use the same network get full network in their phone.)