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:

PSTN:
voice service voip
 allow-connections h323 to h323
!
interface Loopback0
 ip address 177.254.254.254 255.255.255.255
!
dial-peer voice 911 voip
 destination-pattern 911
 session target ipv4:177.254.254.254
 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 permit VoIP call testing, it lack one important feature, available with “real” PSTN phone: placing calls from the PSTN phone to in-rack phones. While this may seem a serious issue, you can always use “csim start” command on the PSTN router to overcome this obstacle. Have fun!