Hey, Fairly new and doing a POC on the 3CX PABX software I noticed a strange thing. I have a phone in a subnet different from the subnet the PABX is in. The client on the phone is cycling between 'registering' and 'timeout'. Doing a packet capture in my network the closest to my phone (port mirror on outgoing interface of firewall) I observe the following: 192.168.2.208 = IOS phone 192.168.88.249 = PABX 1. Client sends SYN for TCP 5001 22:07:45.449914 IP 192.168.2.208.49720 > 192.168.88.249.5001: Flags [S.], seq 2561195304, win 65535, options [mss 1460,nop,wscale 6,nop,nop,TS val 336657334 ecr 0,sackOK,eol], length 0 2. PABX confirms and sends SYN ACK for TCP 5001 22:07:45.449970 IP 192.168.88.249.5001 > 192.168.2.208.49720: Flags [S.], seq 1064058999, ack 2561195305, win 28960, options [mss 1460,sackOK,TS val 21360150 ecr 336657334,nop,wscale 7], length 0 2. PABX didn't recieve ACK so resends SYN ACK for TCP 5001 and on and on... 22:07:46.479507 IP 192.168.88.249.5001 > 192.168.2.208.49720: Flags [S.], seq 1064058999, ack 2561195305, win 28960, options [mss 1460,sackOK,TS val 21360408 ecr 336657334,nop,wscale 7], length 0 The phone is receiving the SYN ACK to create the TCP session, but isn't responding after that? I can make calls from with the 3CX client, but my presence just isn't working. If I disconnect and connect via 4G, it works. I don't have any NAT between these 2 networks. Just simple routing. I also have my PC with a 3CX client in exactly the same network. This isn't having any issues: example data part: 22:13:50.568282 IP 192.168.88.249.5001 > 192.168.2.233.63651: Flags [P.], seq 200:275, ack 1, win 245, length 75 22:13:50.623689 IP 192.168.2.233.63651 > 192.168.88.249.5001: Flags [.], ack 275, win 252, length 0 Any ideas why this is happening? Andy.