ACS SMS returns 401 Unauthorized on POST /sms — regression (previously worked); Email works on same resource/key; AU alphanumeric sender Approved

SUMMARY

All SMS sends from our Azure Communication Services resource (Australia data location)

fail with HTTP 401 Unauthorized. Email on the SAME resource and SAME access key works

fine. Our Australia alphanumeric sender ID is Pre-registered and Approved. This appears

to have worked previously and recently stopped.

FAILING REQUEST

Reproduced from the Azure Portal "Try SMS" (which signs with the resource ACCESS KEY via

HMAC-SHA256 — i.e. independent of any app code or user RBAC), and also via a raw REST call:

POST https://<my-acs-resource>.australia.communication.azure.com/sms***

Auth: <removing PII>

Response: 401 Unauthorized

Body: {"error":{"code":"Unauthorized","message":""}} (message is empty)

Same result on api-version 2021-03-07 and on the newest 2026-01-23.

WHAT I HAVE ALREADY RULED OUT

• Not the application: the Portal "Try SMS" fails identically with a key-signed request.
• Not the access key: Email (POST /emails) on the same resource + same access key succeeds. Only /sms returns 401.
• Not RBAC / Entra: the failing request uses HMAC access-key auth, not a user token.
• Not the sender ID: it is Approved for Australia.
• Not the API version: the 401 response's api-supported-versions header lists 2021-03-07 through 2026-01-23; an unsupported version would return 400, not 401.
• Not the recipient/number/content: the 401 is returned before message validation.

SUBSCRIPTION TYPE

• Offer: "Azure plan" (Microsoft Customer Agreement pay-as-you-go — a standard PAID subscription, not Free Trial / MSDN / Sponsorship). So SMS eligibility is NOT blocked by an ineligible subscription offer type.

QUESTION

• Has anyone seen previously-working ACS SMS begin returning an empty-message 401, and what resolved it?
• Is ACS SMS SEND entitlement separate from alphanumeric sender approval? What would cause a key-signed 401 on /sms while /emails succeeds on the same resource + key?
• Please confirm whether this subscription is CSP / partner-managed, and whether ACS SMS to Australia has any eligibility restriction on this billing/agreement type.
• Given it is a paid "Azure plan" (MCA) subscription with an Approved AU sender and a valid access key (Email works on the same resource/key), please identify why the SMS primitive returns 401 and how to enable it.

Happy to provide resource ID, subscription ID and correlation IDs privately if a

Microsoft engineer needs them.