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:
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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:
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The verstat parameter is used even if associated with an anonymous sip/sips URI.
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If there are multiple PAI headers with verstat information, only the first one is used.
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The attestation level is taken from the verstat parameter if it contains one of the recognized authentication levels A, B and C.
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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).
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If no header provides an attestation level, level A is assumed.
