Posting on their support forums, an O2 representative has said the Windows Phone 8.1/Lumia Cyan OS update for their Lumia handset range is now available for all their WP8 handsets.
If users have not received update notifications he suggests they check for updates manually via the Settings>Phone Update.
He also promised that the visual voice mail update for the Nokia Lumia 930/635 is also on the way, and should arrive in short order.
Read the support page at O2’s forums here.

