Thanks for that - and I may have the answer. As we don't want to take incoming calls on the cellular line (the circuit was put in in order to make outbound calls less expensive, not to make inbound calls more expensive!), I didn't bother setting up an inbound route as we didn't want it to be used that way.
I've now setup an inbound route with an instruction to drop the call and, so far, the card has stayed up for twelve hours. Will test again in the morning...