I think that’s pretty standard with PayG phones - use it or lose it. If you want a phone for occasional use over a long period you need to get one on the cheapest possible contract - I got my Mum one for use in emergencies, with Lebara (who share the Vodafone network) - £3.95 a month.
Which reminds me I need to cancel it now as she is so deaf she can’t use it!