Claim is rejected

I recently put in a claim for my Samsung Galaxy S3..Im a customer for 3years or more but according to them my claim is not valid . Ive noticed during my tea break at work that my phone were ****** out of my handbag in the office while i was out. The insurer say that i cant claim a new phone because my handbag were supposed to be locked in a locker even though we dont have lockers . What a bad pathetic service from 24HOUR MOBILITY !!!! I am paying R122 every month even though my cellphone contract is finish with Mtn !!! What to do?? Fedup customer !!!!!