[T-Mobile] Voice calls are not reaching user devices

Problem

When xMatters tries to call a T-Mobile device, the following error is displayed in the event log:

Notification attempt failed due to LINE_BUSY. Device was busy. The called number returned an ISDNbusy signal, (480 temporarily unavailable, 486 busy here, 603 busy decline).

This error may occur with xMatters events or device validations.

Environment

• All versions of xMatters

• T-Mobile devices

Cause

T-Mobile has a feature called Scam Shield (previously ScamBlock) that may intercept calls that appear to be spam or scams. xMatters calls can be flagged because:

• Calls may be made in rapid succession or be of short duration, which can resemble spam behavior.

• The caller ID used by xMatters can change, or originate frequent calls, which can cause it to be classified as potential spam.

Scam Shield and device-level call-filtering settings can block or silently divert these calls, even if SMS notifications continue to arrive normally.

Resolution

To allow xMatters voice calls to reach T-Mobile devices, try the following steps:

• Review T-Mobile Scam Shield app settings

  1. On the device.

  2. Go to Home → Scam Shield.

  3. Select Manage → Manage scam shield settings → Block Spam categories and review which spam-blocking categories are enabled.

  4. Review the Blocked List and confirm that the xMatters caller ID is not listed.

• Verify T-Mobile account-level settings

  1. Check your T-Mobile Scam Shield settings at https://www.t-mobile.com/benefits/scam-shield.

• Check device call-filter and block settings

  • On the phone, review any built-in call-filtering features (for example, "block unknown callers" or "silence suspected spam"), and adjust them so that calls from the xMatters caller ID are permitted.

• Ask T-Mobile to allow xMatters calls

  • Contact T-Mobile directly and request that calls from the xMatters caller ID be treated as valid and not blocked.

  • Provide the xMatters caller ID, the receiving phone number, and (if possible) the dates and times of recent failed or test calls so T-Mobile can check for account- or network-level blocking.

• Report the xMatters number as valid

  Use T-Mobile's call reporting page to report the number as legitimate: https://callreporting.t-mobile.com/

• If issues persist

  • Work with xMatters support to send additional test calls and review the event and call logs for results such as Connected, Reached Voicemail, NO_ANSWER, or provider failure messages. These details can help T-Mobile identify external blocking or filtering.

In addition, your organization may wish to register commonly used caller IDs with public caller registries recommended by carriers (for example, Free Caller Registry) to reduce the likelihood of spam filtering.