Generally Available: Cleanup legacy AlternateId usage to ensure continued service

We have identified an issue impacting Multi-line (Multiple Number Assignment) feature for Teams Phone enablement related to the usage of AlternateId in customer internal implementations for Azure Communication Services (ACS). AlternateId is an Microsoft internal usage parameter for phone numbers related to Multi-line in Teams Phone. To ensure compatibility with the phone system, AlternateId should not be used by any customer for ACS. We have detected that some customer configurations are currently using non-phone MRI values for AlternateId in internal provisioning workflows. This configuration is not supported and can result in call failures or unintended behavior as Multi-line capabilities are enabled within your tenant.

Action Required:

If your organization is using AlternateId:

• Ensure that usage of AlternateId is ceased immediately
• Continued use of unsupported AlternateId values may result in service disruption.

Temporary Mitigation (If More Time is Needed):

If you are unable to complete the required remediation immediately, you may request to be added to a Multi-line feature blocklist by contacting Microsoft Support.

Being added to this blocklist will temporarily prevent Multi-line enablement for your tenant while you work to correct your AlternateId implementation. Important: This blocklist will expire on June 15, 2026.

If you need assistance with remediation or require temporary blocklisting, please contact Microsoft Support.