[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 ScamBlock that may intercept calls that appear to be spam or scams. Calls that are made in rapid succession or are of short duration can be flagged and blocked by T-Mobile.

Resolution 

Users can verify their T-Mobile device in ScamBlock feature by contacting T-Mobile directly to request an exemption for xMatters calls. Note the call display number, receiver phone number and advise T-Mobile the number for xMatters is valid and should not be blocked.

Alternatively follow this link to report the number as valid: https://callreporting.t-mobile.com/