The "ACK is not received from sip" has been discussed many times but I have not found the definite resolution. My situation is as follows: Setup 1. 3CX Phone System V11 running on Amazon EC2 Win with Elastic IP (Fixed IP). 2. "Turn off STUN requests" enabled in 3CX Network setting. 3. External Extensions setup for 3CXPhone on windows and Aastra 6730i deskphones. 4. 3CX Firewall check passes all checks. Problem: ** Outbound Calls from Aastra 6730i drop after 30 seconds. ** 3CX Server Activity Log shows "ACK is not received from sip:222@<EC2 Elastic Public IP>:5060" ** When making an outbound call from the Aastra the log shows "Extn:222 has joined, contact <sip:email@example.com:1024>" Inbound Calls to all extensions (3CXPhone and Aastra) work fine. Outbound Calls from 3CXPhone on windows, which is on the same LAN as the Aastra 6730i, works fine. Any suggestions.