A feature called SIP Application-Layer Gateway, or SIP ALG, is known to cause issues with VoIP Communication.  Vantage Unified has created this article to assist with properly configuring your Cisco device.

Step-by-step guide

Complete the following steps to properly configure your Cisco device.  Note: These images may differ from the current version of software.

Configuring via Command Line

  1. Connect into the device using a Serial cable, Telnet, or SSH
  2. Enter EXEC mode
  3. Enter Configuration mode
  4. Type the following commands
    1. no ip nat service sip tcp port 5060
    2. no ip nat service sip udp port 5060
  5. Exit Configuration mode
  6. Save the Running Configuration by typing the following command
    1. copy running-config startup-config

Configuring via ASDM (ASA Only)

  1. Connect into the device using ASDM
  2. Select Configuration, Firewall, Service Policy Rules
  3. Open the Global Policy rule
  4. Select Rule Actions, Protocol Inspection
  5. Uncheck SIP
  6. Click OK
  7. Click Apply
  8. Click Save

 

Cisco Configuration Guide

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_nat/configuration/xe-3s/asr1000/nat-xe-3s-asr1k-book/iadnat-fw-sip-alg-hardng.html

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