SIP ALG (Application Layer Gateway) is a feature which is enabled by default in most TP-LINK routers and inspects VoIP traffic as it passes through and modifies the messages on-the-fly.

In few situations this is useful, but in most situations SIP ALG can cause problems using the service. Very often, SIP ALG is poorly implemented causing VoIP messages to be modified in a harmful way or simply corrupts them. In such cases you may experience issues registering your handset, one-way or no audio, unable to receive calls, and issues with Busy Lamp Field (BLF)  and Message Waiting Indicators (MWI).

For our Hosted VoIP service, including out Single User services, SIP ALG must be disabled on your TP-LINK router.

This guide explains how to disable SIP ALG on the following models:

• TP-LINK TD-W8960n
• TP-LINK TD-W8968 (hardware version 4)
• TP-LINK TD-W9970 (hardware version 2)

1. Fire up a browser (Firefox, Chrome etc) and go to 192.168.1.1 (this is the default IP address)
   The default username is admin and the default password is admin
2. On the left, click on Advanced Setup and then click on NAT and then click on ALG
3. Uncheck the box by SIP Enabled
4. Click Save/Apply

Different TP-LINK routers may have different instructions. Please see list below for instructions for your particular device.

Further information can be found at TP-Link TD-W8968 User Guide.