I’ve done a bit more testing and, as far as I can tell, it is network related. I can see the SIP qualify packet is sent by the server. On the client side with Wireshark I can see the packet arrives and we reply. The packet is received on the working server but not on the server where the client shows as unavailable. Does anyone know what network configuration might cause this? This isn’t a case of NAT. Both the server and client addresses are correct. Thank you for your help.