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.
All versions of xMatters On-Demand, T-Mobile devices
Users can whitelist their T-Mobile device in ScamBlock feature by contacting T-Mobile directly to request an exemption for xMatters calls. Use the call display number and advise T-Mobile that this is the originator for xMatters calls.
T-Mobile has a feature called ScamBlock which intercepts calls and prevents completion.