O2 999-day Topup Expiry?

I discovered my 'standby' O2 mobile had been disconnected despite having
made a call within 180 days and when I contacted customer services I was
informed that the last topup I had made was more than 999 days ago and
had therefore expired and that this condition was stated in section 7.1
of the PAYG terms and conditions. However this section only states that
it is only necessary "to make or receive a chargeable call every 180 days"
to avoid disconnection. (How can you *receive* a chargeable call in the
UK?). They did agree to reconnect my number but whether it will be
disconnected in another 999 days is unknown!

Has anyone else come up against this 999-day rule for O2 topup expiry? It
sounds more like a computer software date limitation as I am sure that
somewhere else it states that topups do not expire. Do any other providers
operate a similar 'secret' topup expiry period not mentioned in terms and
conditions? (I do recall very early Vodafone PAYG accounts had to be
topped up quite frequently but my O2 PAYG account is only about 4 years
old.)

RodP