The Importance of Caller ID

Ross Mckillop

Ross Mckillop

9th January 2014

We launched our new SIP stack a little over a year ago now, and despite clear documentation, our own Guidance on Nuisance calls and supporting multiple common methods of sending Caller ID (“CLI”) we have noticed a number of customers that are still sending calls with invalid caller ID.

From today we are implementing new measures to help you comply with the requirement to send CLI, at present we are not – yet – handling this traffic any differently.

Our interoperability documentation clearly states;  “Valid Caller ID must be provided on all calls in E164 format (e.g. 447700900123)”  – furthermore we reserve the right to refuse traffic with an Invalid CLI.


From January 2014 we will start identifying this traffic, and may impose restrictions in the future on any traffic without a valid CLI.  To help you identify any traffic that has an invalid CLI we will return a new X-Warning SIP header in response to any INVITE sent to our SIP platform with invalid CLI.  There are two levels of warning;

  • X-Warning: CLI Should be in E164 Format
    This will be sent where the CLI was interpreted as valid but not in the desired format – e.g. 07700900123 instead of 447700900123 – where possible please send CLI in E164 format.
  • X-Warning: Invalid CLI
    This will be sent where no valid CLI could be found either in your From headers or in any provided P-Asserted-Identity or Remote-Party-ID header.

Look out for the X-Warning headers, log them if you can, and you can use this to identify traffic that may be problematic.

There are a couple types of ‘Invalid’ CLI we see on a regular basis, please ensure your own traffic doesn’t fall into these categories;

Internal Names or Numbers

Within the VoIP world you can send anything you like as CLI,  within an office for instance it’s perfectly acceptable (and a good idea) to send “John Doe” with an internal call so you know who is calling.  Also most offices use 3 or 4 character extension numbers – such as 101, or 2120.

It is, however, imperative that these are not passed to the PSTN – there are penalties that can be enforced against telcos that allow such CLIs to be passed onto the public PSTN. As a wholesale service provider we don’t sanitise your CLI, we expect our customers to be responsible for what they send.

Taking this one step further we occasionally see calls from misconfigured PBXes identifying themselves with the default “asterisk” or “0000000000”

Withheld / Anonymous

Where you do not wish to reveal the calling number (e.g. a “Withheld” or “Anonymous”) call this may be achieved in a number of ways such as P-Asserted-Identity (RFC 3324) or the Remote-Party-ID header however valid underlying CLI must still be provided – the called party won’t see it.

We would remind you that, in accordance with Ofcom’s Nuisance Call Regulations, anyone making outbound sales calls must present a valid CLI that can be called back by the customer.  It is not acceptable to make these calls from withheld numbers under any circumstances.



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