4452d530_able___FreeSWITCH___Confluence__Description
[ITU-T Q.850 Code] 28 [SIP Equiv.] 484 [Enumeration] INVALID_NUMBER_FORMAT [Cause] invalid number format (address incomplete) [Q.850] [Description] 
This cause indicates that the called party cannot be reached because the called party number is not in a valid format or is not complete.
Hangup Cause Code Table - FreeSWITCH - Confluence
Description