This is an intermittent problem: After coming back on the line after having put someone on hold, the caller can hear me just fine, but I have lost audio and cannot hear anything from their end. Has anyone experienced this or know the cause/solution?
I remember we had a similar issue where our Legal team would be on calls where they were listening for extended periods without speaking (e.g. for 20 or 30 minutes), and when they finally did try to speak the other parties on the call could not hear them. I believe the RC team had us increase the TCP Session Time Out on our firewall that the calls were passing through. That may not be a fix for your issue, but it sounds similar so it would be worth a look.