As a rule, you should use the mobile phone once within 12 months.
Whether it's sending an SMS or MMS or even just making a quick call. Otherwise the number AND the credit will expire.
You can also send an SMS with the content STATUS to 444 from your mobile. It costs nothing and the mobile was once again active in the network.
You can also have a deactivated SIM reactivated after the 12 months (within a certain period).
For further Informations, call 0800 800 800.