Caller Line Identity (CLI) Presentation

TTNC allows you to pass on a Caller Line Identity (CLI) number which we pass out onto the UK and international PSTN telephone networks.

If you already use a TTNC number (including a number ported to our network) you're able to assign that number as your outbound CLI using our online control panel, myTTNC.

To restrict fraud and nuisance calls occurring, only 01/02 Local Numbers, 03 UK Wide Numbers and 0800/0808 Freephone Numbers can be used as an outbound CLI.

We don't allow the use of 070, 084, 087 or 09 Numbers under any circumstances.


Using your own number with a TTNC SIP Trunk

When using a TTNC SIP Trunk you're able to register to use a non-TTNC number as the outbound CLI. Before doing so you'll need to complete our CLI Customer Agreement Form.

There are three main types of CLI that are supported by our network:

  • Network Number - This is a valid telephone number which represents the end-user’s network endpoint. This is normally the same as the subscriber number which has been allocated to the line.
  • Presentation Number - This is a used where the caller desires a different number to be seen by the called party on any display technology. This number must always be a valid telephone number that is associated with an end-user. Normally this would be a direct dial for an extension number of a PBX or a non- geographic number associated with the business.
  • Last Diverted Number - Where call diversion is active, this is the network number of the line that performed the call diversion feature. If there has been more than one divert leg, this is always the last one.

This feature will be enabled on your account after you've signed and returned this document to agree with the following:

1. You will only pass CLI to us that is known to be true by you and allocated to the subscriber making the call. This can be achieved by three means:

1.1. The CLI value is assigned by your equipment to the call and overrides any number sent by the user. The value assigned is a telephone number that has been issued by us to you, or you have written proof from the network operator for that number range concerned that it has been allocated to the user or you.

1.2. The CLI value has been obtained directly from a recognised PSTN network operator via a TDM connection using ISDN / SS7 signalling.

1.3. The CLI value has been passed to you by us.

2. You will not pass CLI that you have obtained from other VoIP operators or CLI that the user could control themselves, or that has come from any other ‘untrusted’ source.

3. You accept that any CLI passed outside the UK will be on a ‘best efforts’ basis only and may not be passed to an overseas subscriber

4. You agree that the provision of this CLI Presentation services is on the basis of ‘goodwill’ by us and does not form a contractual part of our standard service

5. You accept that in the event of any complaint received by us caused by any incorrect, false or otherwise confusing presentation of CLI data by you to us, we will have the right to withdraw this option without notice, suspend any other services and retain any cleared funds you currently have with us. Further legal action may also be taken if necessary.


TTNC CLI Customer Agreement Form

Download the CLI Presentation Agreement PDF

Technical note for passing a CLI number

The following SIP headers are used in the definitions below.

From:As described in RFC3261
Remote-Party-ID: (RPID) As described in draft-ietf-sip-privacy-04.txt. The values of privacy=full and privacy=yes are both supported.
P-Asserted-Identity: (PAID). As described in RFC3325 and RFC5876. It is however only recognised in an INVITE request.

Privacy:The value ‘id’ is recognised as described in RFC3325
Diversion: As described in RFC5806

For all the above headers, only the user portion of the URI will be read by our network, and for the avoidance of doubt, this must be in the format .

Any data supplied as textual data will be ignored. Any header which contains characters in the user portion which are not within the range 0-9 or the + character will be discarded.


Our network supports the following different systems:

Network number supplied in PAID header.


  • The network number can be withheld by either the use of the Privacy header or by the text anonymous in the user portion of the From header. If the From header contains numerical data which is different from the PAID header, it will be considered as a Presentation Number
  • The network number can be withheld by the use of the Privacy header. If an RPID header is present and contains numerical data which is different from the PAID header, it will be considered as a Presentation Number.


No PAID header supplied.

Numerical data supplied in an RPID header will be considered as a Network Number.


No PAID or RPID header supplied.

Numerical data supplied in the From header will be considered as a Network Number.



IAX Services

Our IAX services currently only support one Caller Line Identity which will be used as a network number.

This number may be withheld from presentation by the use of either a ‘W’ before the dialled number, or the user of the ‘Withheld’ data descriptor for the calling party number.

Please sign and return by email to sales@ttnc.co.uk, by fax to 0845 468 1001 or by post to: TTNC Limited, 4 Wells Street, Chelmsford, CM1 1HZ.