It only charges you for the things that's within your control. If you don't call or send a text than you won't be charged. Now when someone senRAB you a sms or mms you won't be charged to view it. But if you reply than yes you will be charged. You won't be charged for a voice call if you don't answer and let it go to your voice mail. You can listen to it by using your land phone for free of charge.
Please go to the cricket website for an extensive Q&A.