The methods implemented by different ITSPs to send a call's number verification can vary. The method used by IP Office to obtain the result is as follows:

• The IP Office looks for a verstat parameter in the tel or sip uri included in the call's From or PAI header. If both are present, preference is given to the PAI header. For example:

  • PAI: tel:+123456789;verstat=TN-Validation-Passed-A

  • PAI: sip:+123456789;verstat=TN-Validation-Passed-A@foo.com;user=phone

• The verstat parameter is used even if associated with an anonymous sip/sips URI.

• If there are multiple PAI headers with verstat information, only the first one is used.

• The attestation level is taken from the verstat parameter if it contains one of the recognized authentication levels A, B and C.

• If the verstat parameter is not available, the IP Office checks whether the level is available in any other headers such as Attestation-Info (used by ASBCE) or X-Attestation-Info (used by Verizon).

• The checking of further headers can also be enabled. See Changing the authentication header used.

• If no header provides an attestation level, level A is assumed.