I just installed FS and registered two cisco 7960 with default configuration file. They can talk to each other.  After connecting both phones, I can put one on hold with music, but I can not get the connection back by pressing resume softkey button. Once I pressing resume button, the connection was hangup.

 

I can see the cisco 7960 sent INVITE to freeswith once I press resume button but FS sent back 'SIP/2.0 500 Overlapping Requests ' message. This cause cisco 7960 sent out BYE message.