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New Member

Cisco ISDN Backup For SIP

Greetings, previous to the current project im working on ive only ever used either ISDN or SIP for inbound and outbound call connectivity.

The current prospect im looking at is a single site which will be using SIP as the primary VOIP transport, SDSL for Data and ISDN as a backup, configuration of this setup isnt a problem but there is one issue i haven't thought of before.

For example a block of DDI's will be assigned to the incomming SIP trunk's. In this scenario lets say they are the following:


0121 872 0211


0121 872 0212

0121 872 0213

0121 872 0214

0121 872 0215

0121 872 0216

0121 872 0217

Could someone let me know how i would achieve fault tolerance using an ISDN connection should the WAN connection drop by continuing to allow for incomming calls using the existing DDI range as of course the SIP trunk and ISDN connection are two seperate entities.


Hall of Fame Super Gold

Re: Cisco ISDN Backup For SIP

If you have both routers under you control you can configure isdn as backup, and SIP connections would work anyway.

Hope this helps, please rate post if it does!

New Member

Re: Cisco ISDN Backup For SIP

We would have control of the customer equipment but not the service provider.

Its a tricky one as im unsure how the incomming main number would be switched over to ISDN when the SIP connection drops out.

For example customers may ring 0121 872 0211 as the primary number for the business, this is associated too the SIP trunk, when the WAN connection dies and the SIP trunk is lost how then would the number 0121 872 0211 be transferred switched too the ISDN network transparently whilst the SIP trunk is down?

Hall of Fame Super Gold

Re: Cisco ISDN Backup For SIP

That is what I was trying to say, you cannot transit the call on ISDN natively because the numbers are different, the only thing you can do is an IASD data backup, but you would need to terminate it in some place that nevers goes down ,and to which you ITSP can reroute SIP calls.