When you say it has stopped working is this because of non use? ie. Has it been over 90 days since you used it?
If so, I'm afraid there is no way to get that number back.
From terms and conditions
3.8 Keep talking!: You need to use our Services at least once within any 90 day period either by making a chargeable call or sending a text message. If you don’t we may suspend your access to the Services and cancel this Agreement. If we cancel this Agreement, you may lose your phone number and any unused Top Up credit on your account (see Clause 10.3). We don’t really want to lose you as a customer so please ensure you keep talking or texting!
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