It'll get deactivated after 6 months and completely disconnected after 13 months.
See here:
7.2 In addition to the circumstances set out in Section 7.1 above, we may suspend (i.e. bar), restrict or end the provision of the 48 Services (in total or in part) if you choose not to purchase top up credit or make a chargeable use in respect of your Account for a period of 6 months.
7.3 Where the 48 Services are suspended because you have chosen not to purchase top up credit or make a chargeable use in respect of your Account for a period of 6 months, we will remove any credit on your Account after a further 2 months and credit will not be recoverable. Your service will then remain suspended for a further 5 months and can be reconnected at any time during this period. When the suspension period ends your service will be disconnected, this agreement will end and your number will be immediately lost.
7.4 We will try and tell you when we suspend, restrict or end your use of the 48 Services but we are not required to.
If you need to contact 48 support about this then see the links in my signature below.
