c52b1065_User_Guide___Groupcall_Support__Description
[Message Status] Messages sent to unreachable recipient [Notes] This status, depending on the network that is used, can be returned for the same error. [Description] 
Mobile Number deemed as unreachable due to a variety of factors. Number can be invalid (but correctly formatted) or other.
Messenger v5 User Guide | Groupcall Support
Description