SIP Trunk 403 Forbidden on REGISTER with Premium App Service Account

Getting a 403 Forbidden when our AppFoundry Premium App service account attempts to REGISTER with a configured SIP trunk via the Platform API. The credentials are valid for manual testing, but the automated integration fails consistently.

api_version: v2
endpoint: /api/v2/telephony/phone-features/registrations
method: POST
payload:
 authId: "service_account_id"
 authSecret: "secret"
 sipAddress: "sip.trunk.provider.com"
 port: 5060

Is there a specific permission scope missing for programmatic SIP registration in a multi-tenant environment?