Mandatory CLI Changes

Ross Mckillop

Ross Mckillop

6th September 2018

As we have previously announced, there are big changes to CLI coming with the new Ofcom General Conditions which take effect from 1st October 2018.

Ahead of this date, all customers must ensure they are sending valid Caller ID on all calls, or ensure that the trunk these calls use has a Default CLI set.

Calls that do not have valid Caller ID will not complete.

Two Caller IDs?

As Neil explained well in his recent article there are actually two Caller ID numbers transmitted with every call; a Network Number and a Presentation Number. Traditionally we have treated these equivalently, and called them simply “Caller ID”

For customers that require it, we will expose both of these and allow you to set different values for both subject to various rules which are explained below.

The Network Number translates to the P-Asserted-Identity header in SIP. Whereas the Presentation Number will be derived from the From header where present and valid.

Setting a Network Number

The Network Number (P-Asserted-Identity in SIP) is mandatory and must be a number on your Simwood account, that would allow the called party to place a return call. Even where the CLI is to be withheld to the end user, this header must be present and contain a valid number.

The API and Portal will soon be updated to allow a default Network Number per trunk in much the same way as the current Default CLI option.

To facilitate an easier change for many of our customers, we’ll accept non-Simwood numbers in the P-Asserted-Identity as long as they are valid (that is they are of a valid length and, if a UK number, are allocated to a CP) however you should be aware that these will be sent as a Presentation Number, and the Network Number will be overwritten with a generic one from our 08979 range (for more on these numbers, see below) or the trunk default, if set.

Setting a Presentation Number

There are cases where you or your customers may need to present a number other than their own, we cater for this by allowing you to set the presentation number.

The most common reason is because the caller is acting on behalf of someone else, e.g. a callcentre calling on behalf of a company can legitimately present a main contact number for that company that the called party will recognise (Ofcom consider this a Type 5 Presentation Number), or where calls are forwarded you may wish to pass along the original caller’s CLI (this is termed Type 4)

The Presentation Number should be set in user portion of the SIP From header.


Ofcom has allocated a new range for Inserted Network Numbers starting 08979. A number starting 08979 will be inserted where no valid Network Number has been provided.

It’s unfortunate that these numbers bear a resemblance to PRS numbering, however these numbers should never be displayed to an end user, and cannot be called back.

Given that we expect there to be a period of transition for many operators, we would anticipate that the 08979 number may occasionally be “leaked” to some destinations unintentionally and we would advise that you ensure your support teams are aware of these new numbers and able to field enquiries from your users should they arise.

Why are you changing this?

It’s not just us, these changes are how we’re complying with the new General Conditions which all CPs are bound to comply with. We support what Ofcom are trying to achieve here by restoring confidence in CLI.

All CPs will be required to take measures to ensure the authenticity of CLIs on their network, this isn’t exclusive to Simwood. The technical implementation may differ from provider to provider, and some will mask the concept of Network Number and Presentation Number, likely by allowing their customers only to set the Presentation Number, but we offer an interconnect service to other CPs only, and aim to offer as much control over the signalling as possible.


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