srp521w fxo relay

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Jan 18th, 2010
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Dear All,

Can anyone describe how can we use the FXO relay function on SRP521W?

Does it relay PSTN incoming calls to one of the FXS ports all the time, or is it configurable?

What about connection between FXS and FXO relay when we have power failure, does PSTN backup to one of the ports like SPA3102 does?

Best Regards,


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Andrew Hickman Wed, 05/19/2010 - 03:00
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  • Cisco Employee,

Sorry for the late response:

A relay switch is incorporated in to the SRP520, such that it directly connects the FXO port and the first FXS port when:

  • There is no power supplied to the SRP
  • The device is powered, but the is no active SIP registration

Therefore, when the device is actively registered to a SIP telephony service, the FXO port is disconnected and otherwise unused.  Because the FXO port is only ever used to directly connect a phone line and a handset/fax connected to the first FXS port, there is no configuration required for it.

It is not possible to route calls to the FXO port on the SRP520.

Safar Safarov Thu, 11/11/2010 - 14:12
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does it mean that it's impossible to use SRP520 as a micro PBX (for two-three users for example)?


Andrew Hickman Thu, 11/11/2010 - 15:25
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  • Cisco Employee,

Correct - the FXO port is not really active in any way - think of it as a basic pair of copper wires that connect phone port 1 to the jack in the wall in failure conditions (i.e. equivalent to unplugging the phone and plugging it directly in to the wall jack)


Safar Safarov Thu, 11/11/2010 - 21:57
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Many thanks for your reply.

Is there any Cisco SOHO product that can provide such feature?