CME - Configuring the FXS port for a POTS phone


In order to connect a POTS phone to a CME router, you require an FXS module. Fortunately, they are very affordable and readily available on ebay. On my routers, the first FXS port is located in chassis 0, module 1 and port 0 or 0/1/0. The second at 0/1/1. Your configuration may be different.

On some Cisco voice platforms, one or more Packet Voice and Data Modules (PVDMs) are needed to support the voice interface cards. If no PVDMs are present, the voice ports may be missing in the active configuration. The PVDMs hold digital signal processors (DSP farms) that make the VICs fully functional, and are installed on the motherboard of the Cisco router series or on the network modules. The PVDMs are rated by the number of DSP channels that they have: 8, 16, 32, 48 and 64.

To check if you have a PVDM module, issue the "show diag" command. On 2811 routers, there are two PVDM status LEDs on the front lower right corner that indicate the presence (lit) or absence (dim) of PVDM modules.

Typical dual channel FXS module.

Typical POTS phone (corded phone).

Connect an analog POTS phone to your router. Connect it to FXS port 0/1/0. From global config mode, you need to define a POTS dial peer. This is done with the command:

Router(config)# dial-peer voice 1 pots

Next, you need to define the extension this device will use. Using the destination-pattern command, assign it the extension 5001. I gave the POTS phone distinct extension numbers from the IP phones to aid in troubleshooting.

Router(config-dial-peer)# destination-pattern 5001

Finally, tell the router which voice port to use. This is done with the port command.

Route(config-dial-peer)#  port 0/1/0
Route(config-dial-peer)#  no shut

To configure another port, create another dial-peer but make it number 2: "dial-peer voice 2 pots". Give it a unique extension and use the other port 0/1/1.

Cisco Pecularity: When you connect a POTS phone into an FXS port, even if the port is not configured, you will hear dial-tone AND can dial out to the other phones. You will not be able to dial to that phone until an extension is configured for the port.

Use the "show voice port summary" to reveal the ports and their line status. Test the functionality of this dial peer by placing and receiving calls to/from your VoIP phones.

Hardware Requirements

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Copyright July 2013 Eugene Blanchard