ISDN Number Presentation

I was on an install recently that involved deploying a Sonus SBC 1000 to replace an upstream Mitel 3300 ICP and a Dialogic Media Gateway – effectively this was to retire old and unnecessary hardware so that Lync can connect directly to ISDN (via the Sonus).

There was a snag when trying to present the calling number on the ISDN line. The line had Calling Line Identification Presentation (CLIP) No Screening enabled which allows presentation of any calling number on an outbound call. And the client confirmed that previously any number could be displayed, even a single ‘9’ for example. So the fact that the DDI wasn’t coming up at all (even incorrectly) was a bit strange. It just kept displaying the pilot number no matter the format presented.

The Sonus was sending out the Calling Party Number Type and Calling Party Number Plan both as Unknown. It was necessary to set the Calling Party Number Type to ISDN and the Calling Party Number Plan to National.

The following table shows the Calling Party Number Information Element of the Q.931 protocol.

Q.931 Calling Party NumberAs you can see the number to be presented is prefixed with an octet that describes the format of the subsequent number digits. Just as E.164 states that the prefix “+” means the following number includes the country code (e.g. 44 = UK).

The ‘Type of Number’ bits (7, 6, and 5) are defined as…

Binary Name
0 0 0 Unknown… is used when the user or the network has no knowledge of the type of number, e.g. international number, national number, etc. In this case, the number digits field is organized according to the network dialling plan; e.g. prefix or escape digits might be present.
0 0 1 International Number
Prefix or escape digits shall not be included.
0 1 0 National Number
Prefix or escape digits shall not be included.
0 1 1 Network Specific Number… is used to indicate administration/service number specific to the serving network, e.g. used to access an operator.

* There are more options than this, but you get the idea.

 

The ‘Numbering Plan Identification’ bits (4, 3, 2, and 1) are defined as…

Binary Name
0 0 0 Unknown… is used when the user or network has no knowledge of the numbering plan. In this case, the number digits field is organized according to the network dialling plan, e.g. prefix or escape digits might be present.
0 0 1 ISDN/Telephony Numbering Plan
ITU-T Recommendation E.164
0 1 0 Data Numbering Plan
ITU-T Recommendation X.121
0 1 1 Telex Numbering Plan
ITU-T Recommendation F.69

* There are more options than this, but again, you get the idea.

If you think of a telephone number, starting with the following digits…

349567…

Is that a local 6 digit number in the current area code, or is it the start of a Spanish telephone number. The moral of the story is.. if you saw the Calling Number Type and Plan then you would know. And so would your ISDN Provider.

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About Graham Cropley

Working as a Senior Consultant for Skype for Business, Exchange, and Office 365.

One Comment

  1. And what a fun “retire[ment of] old and unnecessary hardware” that was. 😀

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