[Users] one way audio

Hi all, Anyone experiences an one way audio problem when a call is connected to the called party? I am using openser with mediaproxy to provide services to our customer. However, some of our customer complain about the one way audio problem (calling party can hear called party but not vice versa). The system works to most of the customer without that problem. I wonder why there are some customers experience that problem. I assume there is the port problem during RTP packet transmission. Am I right? In the mediaproxy setting, the portRange is 2000:8000. Do I need to increase it? Any method to trace the problem if my assumption is wrong? Thanks.