Scope:

The following steps will show you how to check a call trace if there is SIP ALG or Double NAT is occurring on the network. SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable. This can give you unexpected behavior, such as phones not registering and incoming calls failing.

Requirements:

  • Call Trace (Preferrably Inbound) - Find out how to obtain one here

Inbound Call Trace

  1. Open the call trace link in any browser
  2. Search for the first INVITE on the right side of the frame. The INVITE should show the extension@privateIP as shown below. If the public IP address is shown instead, SIP ALG or Double NAT is enabled

Outbound Call Trace

  1. Open the call trace link in any browser
  2. On the right hand window, check the Contact line. The Contact should show the extension@privateIP as shown below. If the public IP address is shown instead, SIP ALG or Double NAT is enabled