Collect Call Block with ISDN - It`s possible?

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Sep 10th, 2010
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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?

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