Just an update to the thread... After liaising with plambrechtsen, looks like the account in my situation may have been terminated due to inactivity and not to a Hack n' spam incident as I originally thought in my original reply. My profuse apologies if that the case. (The termination happening to coincide with the Yahoo Email Security issues).
The Yahoo Xtra ID looks like it may have became de-linked from the main Telecom account upon transfer of Account holder (where a new acct number was issued etc). The Yahoo Xtra ID it was discovered was still in the name of the original account holder. It may have been assumed that the Yahoo Xtra ID no longer belonged to an active Telecom account, and was hence terminated.
Many Thanks to Peter and the Telecom helpdesk for following this up.