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Walkthrough Wednesdays

Collect Call Block with ISDN - It`s possible?

Hello gentlemen

Inside I know so far, to block collect calls on digital trunking with Cisco Voice Gatewais there are some options:
    * To request to Telco to do this block

          Some Telco does not do this type of service
          Some Telco charge for this service
          The time used by the TELCO to DO and UNDO this changes use to be big.
    * By using the R2-Digital Double-Answer
          Brings costs to the caller. The time between the arrival time of the call(first answer) until the extension effectively answer the call(second answer) is considered on charges. (This depends, changes from Telco to Telco)
          This service tends to be problematic.

          It's a no so good, and rude, workaround (my opinion)

My question is: And with ISDN? How to block calls?
On the last PVT of Unified communications in Brazil, some people commented of to do this block do with TCL scripts.

I do not know much about TCL, but I imagine that logic is similar to what we did in asterisk world in my last job.
In the link below the asterisker made two dumps of call, one normal call and another collect call.
Comparing what has changed, was found the parameter "Reverse Charging Indication."
When the incoming link present such a parameter, drops the connection.
(Portuguese Brasil)

I decided to follow the Zanolla`s idea and I made the debugs of two calls on a Cisco 2651XM with an E1 on ISDN, one normal call and another collect call.

This link has the output of debug ->

I could verify that the variable that differentiates a collect call from a normal connection is as follows:
        Unrecognized VARIABLE (0x4A) i = 0x81

PS: I believe that suddenly with a newer IOS that "UNECOGNIZED" could turn a known message, I cannot test it now.
Production environment.

Now the first mystery is how to filter it with TCL Scripts.
For this job I ask the help of colleagues

Once you catch what call is collect and what is not, I was wondering what to do:
* Simply block the call?
    * Testing the Calling Party in any way? Maybe a list inside de TCL?
    * Place a digit in front of Called Party to differentiate those calls and work it with on the Call-Manager with the Calling Search Spaces, Route Partitions and else?


Hi Douglas, did you find any answer. We have the same question. 



You can easily block unwanted calls from the PSTN on
your ISDN gateway.

You need to know exactly the digits you receive on the incoming
call from the CLI.
(something like "debug isdn q931" may help)

In this example I want to block calls fron caller 0123 345 7890

voice translation-rule 666
rule 1 reject /0123456789/
voice translation-profile PEST-CALL-BLOCK
translate calling 666
dial-peer voice 1 pots
call-block translation-profile incoming PEST-CALL-BLOCK
call-block disconnect-cause incoming call-reject
incoming called-number .
fax rate disable
port 0/0/1:15

I hope these guide lines can help you.


Regards, Alex. Please rate useful posts.

Hi Alex, thanks for your answer.

We need to block all the collect calls from around the world. In this case do we need to ask Telco to do the configuration in their ISDN gateway or could we do something in a C3900-SPE100 ?


Thank you !!




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