Avis bad customer treatment

I was using Avis car rental at OR Tambo. I believe I dropped my phone on their car since I was using for navigation and realised in 10 minutes after. When I got to the car my phone was not there and their level of assistance when I told them was very poor.The level of customer services was very poor, they promised to call me back but never did in 4 days. I think they didn't take my issues seriously. I was so desapointed with them cos they could have investigated and keep me informed of the progress, so easy.