The below is what is happening with Sangoma S505 connected to Freepbx14. New installation.
First call comes in. The phone answers the call and they are having a conversation.
Second call comes in. The phone gets the call waiting tone but does not answer the second call. As soon as the second call goes to voicemail, there is no audio on the first call. The first call is still shown as active, the timer goes on, but no audio.
Very strange. Can you get an asterisk CLI trace of what shows up during that call scenario? Is the s505 on the same local subnet as the pbx? Also, it may not matter, but can you let us know the firmware version that is on your Sangoma S505?