Call Waiting problem

When my wife is talking to her friend and she gets the call waiting beep, she presses the flash button and is able to talk with the second party. When she flashes back to the previous caller she cannot hear them but the caller can hear her. This occurs with all callers that have called and I've tried 3 different phones and also a portable, which all create the same problem. I have the Grandstream 502 and its behind a Linksys router which I have port forwarded 5000-65000 and still no good. Was hoping someone may know whats going on before I call support. Thanks for any help.

All I can contribute to this is that call waiting work for us. May be an isolated incident with the 502 or your router. I suggest the support route.

What is the Model of the Linksys router? Does it have a SIP ALG?
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My router model is BEFW11S4 V2 and I don't know if I have SIP ALG. I'm at work right now but will check when I get home.