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With G.729, the second line on the Cisco ATA 186 registers but cannot be used to make calls

Core Issue

In simultaneous dual port operation, when the selected codec is G.729a, it is available to only one of the two Foreign Exchange Station (FXS) ports on a first-come-first-served basis. The port for which G.729 is available is configurable. The first port is available by default. The other port on a Cisco Analog Telephone Adaptor (ATA) is limited to G.711 because the Cisco ATA 186 has only one G.729a codec.


To resolve this issue, perform these steps:

  1. Access the Cisco ATA 186 configuration screen from a web browser by entering the URL http://ip_address_of_ata/dev, where ip_address_of_ata is the IP address of the ATA 186 you are registering.
  2. If the TXCodec and RxCodec are set to 3=G.729a, then the Low Bit Rate (LBR) codec must be set to 3.

    Setting the LBR codec to 3 means that G.729a is available to one of the two FXS ports on the first-come-first-served basis. The other port can be configured to negotiate the use of G.711.

    Note: The default configuration is for one port to use G.729 and the other port to fail. This may not be desirable for the second call to automatically use G.711 because of bandwidth consideration. To allow the second call to be successful, a codec class must be configured on the gateway to negotiate the second call using the G.711 uLaw or the G.711 aLaw.

  3. Determine which port uses G.729 by configuring bit 21 of the ConnectMode parameter. Setting this parameter to Ox90 200400 makes G.729 available to the second port. G.729 is available to the first port by default.

As an alternative, set the RxCodec and TxCodec to G.723 and all calls can be supported simultaneously with this LBR codec.

For more information, refer to these documents: