A voice lab rack usually utilizes dedicated piece of hardware to simulate PSTN switch. Commonly, you can find a Cisco router in this role, with a number of E1/T1 cards set to emulate ISDN network side. It perfectly suits the function, switching ISDN connections between the endpoints. Additionally, it is often required to have an “independent” PSTN phone connected to the PSTN switch, in order to represent “outside” dialing patterns - such as 911, 999, 411 1-800/900 numbers. The most obvious way to do this is to enable a CallManager Express on the PSTN router, and register either hardware IP Phone or any of IP Soft-phones (such as IP Blue or CIPC) with the CME system.

However, there is another way to accomplish the same goal using IOS functionality solely. It relies on the IP-to-IP gateway feature, called “RTP loopback” session target. It is intended to be used for VoIP call testing, but could be easily utilized to loopback incoming PSTN calls to themselves. Let’s say we want PSTN router to respond to incoming calls to an emergency number 911. Here is how a configuration would look like:

voice service voip
allow-connections h323 to h323
interface Loopback0
ip address
dial-peer voice 911 voip
destination-pattern 911
session target ipv4:
incoming called-number 999
tech-prefix 1#
dial-peer voice 1911 voip
destination-pattern 1#911
session target loopback:rtp
incoming called-number 1#911

The trick is that only IP-to-IP calls could be looped back. Because of that, we need to redirect the incoming PSTN call to the router itself first, in order to establish an incoming VoIP call leg.

While this approach permits VoIP call testing, it lacks one important feature, available with the “real” PSTN phone: placing calls from the PSTN phone to the in-rack phones. However, you can always use “csim start” command on the PSTN router to overcome this obstacle. Have fun!

Petr Lapukhov, 4xCCIE/CCDE
About Petr Lapukhov, 4xCCIE/CCDE

Petr Lapukhov has more than 12 years of experience working with Cisco Systems products. Not only is he the only person in the world to have earned four CCIEs (Routing & Switching, Security, Service Provider, and Voice) in just two years, he also passed every exam the first time. He shares his knowledge and experience with INE’s students through our various products and programs. Petr works with all of the technologies covered within his four CCIE tracks on a daily basis, staying current with any changes in the industry. He has also received his Cisco Certified Design Expert (CCDE) certification, joining a small group of distinguished individuals who have achieved this status. Petr is a contributor to INE’s blog and our INE IEOC Community Forum. You may contact Petr Lapukhov at

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