Virgin Mobile iPhone customers you get poor call connections-many drops

I am a first time iPhone user... I recently purchased a MacBook Air and love it, and thought it was only natural to puchase an iPhone.  So, I just purchased a no contract iPhone 4 from Virgin Mobile and use their Beyond Talk unlimited Data and messaging plan.  Call connectivity is very poor (dropped calls, poor audio...).  I checked their coverage map and it says I should get average to good performance where I am using the phone.  I understand that Virgin Mobile uses Sprint's network.

So, the question is... Is there anything that I can ask Virgin Mobile to do? or ask Apple? or is it that Virgin Mobile's (Sprint's) network is a poor performer....?