The mobile network provider RBB’s phone is connected to confirms that (if the phone is pay as you go) in order for the SIM to stay active, someone needs to perform some kind of chargeable usage (a phone call or a text message, something like that, just topping up the credit should be adequate too).
In other words - we already knew this, but - someone is definitely still maintaining the number, nearly 18 months after OTRA ended and over a year since the phone was switched on for any period of time. (Either by paying a monthly bill or by switching it on and topping up or sending a text.)
[if you want to know why I think the phone is pay as you go and not on a monthly contract, click here. If anyone wants more information, send me an ask. I think I might rewrite my thoughts on that later on anyway.]