Evidence that a caller or provider actually holds the right to use a telephone number, used to justify full attestation and to support delegated signing.
Full attestation depends on the signing provider knowing that its customer is entitled to the calling number. When the provider assigned the number itself, that knowledge comes from its own inventory. When an enterprise brings numbers from another carrier, the provider needs some other proof that the customer legitimately controls them.
Proof of possession can take several forms: letters of authorization, carrier-issued documentation, verification calls or texts to the number, or cryptographic mechanisms. The IETF has defined delegate certificates (RFC 9060) and ACME-based authority tokens that let a number holder demonstrate control over specific numbers or ranges and obtain a certificate scoped to them.
Whatever the method, the evidence should be recorded and periodically rechecked. Numbers are ported, disconnected, and reassigned, so a customer that legitimately held a number last year may not hold it today. A practical approach is to tie proof-of-possession checks to onboarding, to any new number a customer adds, and to a periodic review cycle, with the result stored alongside the customer record your signing service consults.
Why it matters to carriers
Many attestation disputes come down to whether the originating provider could show that its customer had a right to the number. A documented proof-of-possession process lets you assign A attestation to legitimate enterprise traffic with confidence, and gives you a defensible record if a traceback or regulator later asks why you vouched for a call.