Configure and Troubleshoot Call Forward to the PSTN using SIP Trunks

Posted: April 20, 2012 in SIP

Introduction

With the increased usage of mobility features and work from home scenarios, there is increased demand for the ability to transfer incoming calls from the PSTN back out to different PSTN destinations. When using SIP trunks for PSTN connectivity these calling scenarios often fail due to call validation. This document is a guide to configuration and troubleshooting of these call flows.

Core Issue

All outbound PSTN SIP calls are validated by the SIP Trunk provider to ensure the calls are valid and not toll fraud attempts. The methods of call validation can vary from provider to provider, and many involve multiple methods for the same call. This becomes a concern when using the default settings in a CUCM w/CUBE topology and attempting to call forward an inbound PSTN call back out to the PSTN. One of the most common validation methods is for the SIP provider to examine the “From” field in the incoming INVITE of a call and make sure it matches to a known DID number for that customer. The default setting in CUCM for forwarding calls is to maintain the CLID of the calling originator. This causes the “From” field of an outgoing INVITE to contain the CLID number of the original PSTN caller, and not a valid DID belonging to the customer. When the provider sees this with no additional information, it typically will reject the call setup attempt or ignore it completely.

Resolution

There are three common solutions to this issue.

The first is to alter the “From” field so that the CUCM will send the information of the redirecting number instead of the call originator. In this scenario the “From” field will contain a valid customer DID number, and the call will validate. The advantage of this technique is that it works with virtually all SIP providers. The disadvantage is that the receiving party of the forwarded call will only see the caller-ID of the number that was set to forward (typically their own office DID number). They will not see the caller-ID of the original caller, and therefore will not know who is calling prior to answering the call.

1 – Here are the steps to use the first method:

-Navigate to the SIP Trunk page in CUCM
-Navigate to “Outbound Calls” section of the page
-Change the value in the drop down entitled “Calling Party Selection” to “Last Redirect Number (External)”
-Save the change, and apply the change (may drop calls why you press apply, so be careful with this change during business hours)

A second method is also often used, though it requires the SIP provider to validate calls through a different method. This technique involves adding a p-asserted identity line in the outbound INVITE containing a valid DID. The advantage of this technique is that the “From” field retains the calling originator’s information so that the recipient of the call can see who is calling on their caller-ID. The disadvantage of this technique is that it is not universally used, and customers need to check with their SIP providers to make sure they will validate calls based on the p-asserted identity value instead of the “from” field value.

2 – Here are the steps to use the second technique when using a CUBE gateway:

-Log into the CUBE router
-Add the following SIP profile (make sure to use a unique profile number if you have existing SIP profiles, or add it to your outbound SIP profile in use. Also make sure to use a valid DID number and SBC URL information.)

voice class sip-profiles 1
request INVITE sip-header P-Asserted-Identity add “P-Asserted Identity:<sip:1111111111@sbc.sipprovider.com>”

-Add the following configuration line to the dial-peer being matched to place the outbound call to the SIP provider:

voice-class sip profiles 1

Make sure that the SIP Profile contains a valid DID with your SIP provider. Also, most providers will look for a fully qualified domain name in the PAI, as opposed to an IP address. Be use to use the FQDN in the profile, even if you are routing the calls directly to the provider’s IP address.

The third method is similar to the second but instead of a PAI line, this uses a Diversion Header. The Diversion Header is added to the outbound INVITE and contains a DID number that the provider can validate the call with. Like with the second option, the advantage of this technique is that the “From” field retains the calling originator’s information so that the recipient of the call can see who is calling on their caller-ID. The disadvantage of this technique is that it is not universally used, and customers need to check with their SIP providers to make sure they will validate calls based on the diversion header value instead of the “from” field value.

3 – Here are the steps to use the third technique when using a CUBE gateway:

-Navigate to SIP Trunk page in CUCM
-Navigate to “Outbound Calls” section of the page
-Check the “Redirecting Diversion Header Delivery – Outbound” Check box
-Save the change, and apply the change (may drop calls why you press apply, so be careful with this change during business hours)
-Log into the CUBE router
-Add the following SIP profile (make sure to use a unique profile number if you have existing SIP profiles, or add it to your outbound SIP profile in use. Also make sure to use a valid DID number and SBC URL information.)

voice class sip-profiles 1
request INVITE sip-header Diversion modify “<sip:(.*)@(.*)>” “<sip:1111111111@sbc.sipprovider.com>”

-Add the following configuration line to the dial-peer being matched to place the outbound call to the SIP provider:

voice-class sip profiles 1

Make sure that the SIP Profile contains a valid DID with your SIP provider. Also, most providers will look for a fully qualified domain name in the diversion header, as opposed to an IP address. Be use to use the FQDN in the profile, even if you are routing the calls directly to the provider’s IP address.

Troubleshooting

The following debugs are useful in this call scenario:

debug ccsip messages
debug voip ccapi inout

When reading these debugs keeping the different call legs straight can get confusing. It is recommended to use an application such as Notepad ++, and to mark the Call ID of each call leg traversing the CUBE. The goal of these debugs is to check that the outbound INVITE for the forwarded call contains either the correct “from” field, or the correct “p-asserted identity” or “diversion header” field. If using the “from” field method, check that the SIP INVITE coming from the CUCM contains the full 10 digit DID number for the last redirecting party, and that this value is the same in the INVITE going to the provider. If using either the “p-asserted identity” method or the “diversion header” method, make sure that the outgoing call is matching the correct dial-peer that has the voice-class sip profile set on it. If it is matching a different dial-peer, add the sip profile to that dial-peer.

Related Information where I got this details

SIP Profiles on CUBE Configuration Guide
http://www.cisco.com/en/US/products/sw/voicesw/ps5640/products_configuration_example09186a0080982499.shtml

CUCM Trunk Configuration – Search for “Calling Party Selection”
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/8_5_1/ccmcfg/b06trunk.html

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Comments
  1. Quintin Hassell says:

    Awesome explanation. Just what we were looking for.

  2. Sofos says:

    Hello and thank you for your great post. How would you resolve this if you have multiple DID’s?
    Thanks

    • Network Experts Blog says:

      Thx it wasn’t my post I ran into this issue so thought to share it, however you can play with multiple DID’s for example outbound INVITE containing a valid DID that the “From” field retains the calling originator’s information so that the recipient of the call can see who is calling on their caller-ID in that way you can play around with each DID or multiple range DID

  3. Randle says:

    Ive solved so many issues by stumbling on to this site. Thank you!!

  4. Laszlo says:

    You saved my day 🙂

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