It is recommended to make a chargeable connection or top up at least every 90 days @Treasurer , Vodafone should sent a text notification to remind you to make a connection to the network to avoid disconnection.
You need to refer to the T & C's under suspension, disconnection here: PAYG Terms and Conditions
edited to add after reply from @Treasurer Vodafone will send a text notification at 90 days followed by a further message 90 days later, you should also receive a further text 40 days later and if you fail to take any action you will lose the number and credit. To be on the safe side @Treasurer use the phone after 90 days. There have been too many threads on the forum where numbers have been lost and the poster is desperate to get the number back.
