ShoreTel and PSTN Failover

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ShoreTel Daily Factoid – PSTN Failover

PSTN (Public Switches Telephone Network) failover allows for calls to be placed out of the PSTN in the event of a WAN link outage, or if the inter-site bandwidth has been exceeded.

PSTN failover is a backup to internal and transferred calls in the case of a WAN outage.

If the WAN is out instead of the ShoreTel reporting, “That extension is unavailable,” from a remote site with no DVS or going directly to voicemail at the HQ site or remote site with DVS, the call will be routed out the trunks to the assigned DID, External Number or Menu configured for the users extension.

This process is mostly transparent to customers, and users when set to route directly to the users DID number.

In a PSTN failover situation (Such as a wan outage or out of inter-site bandwidth) the ShoreTel system will use a Blind Transfer (Just Sends the Digits and Connects the Caller) but if for some reason the call routes directly to the users Voice Mailbox, then you will need to select a Consultative Transfer (Send Digits and open a talk path between the two internal parties so that the call can be announced).


In the Class of Service, PSTN Failover must be checked to allow failover for those users extensions at a given site.

In the User configuration screen there is a PSTN Failover field that allows for three options:

  • DID – When DID is selected and a call fails, the call is routed via the PSTN network to a local PSTN DID number for the user.
  • Specific phone number – When a specific phone number is to be entered, a text box opens. This field supplies the PSTN number to be dialed to complete a failed site-to-site call.

When an external number is supplied and a call fails to go directly to its intended extension, the call is routed to the number indicated, for example, an auto-attendant. When reaching an auto-attendant users will be required to dial an extension or operator.

  • None – If there is no available bandwidth or if a WAN is down for a site to site call and if the call destination has no PSTN failover, the call is directed to voice mail.

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