This is just a suggestion, but you might want to normalize that table into two separate tables: TicketMaster and TicketMessages.

TicketMaster
id (int)
inserted (smalldatetime)
lastUpdated (smalldatetime) DEFAULT GetDate()
ticket_id (int)
owner (nvarchar)
subject(nvarchar)
details(nvarchar)
category (nvarchar)
status (nvarchar)

TicketMessages
UniqueID(int) --or bigint if there is a lot of messaging and you need to retain conversions for a long time
ticket_id (int)
message(nvarchar)
postedBy(nvarchar) --or a user id
postedOn(smalldatetime)

Each message in TicketMessages has a unique id in case you need to delete a message. This allows you a little more information on the conversation which can help. The other setup doesn't tell which is the primary record for all the messages. Plus it also tells you who posted the last message instead of leaving it up to the user to post a signature. I changed the ticket_id column to an integer, you may require alpha characters in your ticket ids so I didn't think about that until after the fact. But if you don't need the alpha characters then you can use the identity feature to auto increment that is built into sql.