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I suggest you refer the matter to the telecommunications ombudsman.
Whether or not they can order that your old number be referred will depend on whether it has been allocated to another party, but as an alternate they may be ordered to pay compensation for any damage to your business arising out of their loss of your number.
Naturally, your entitlement to compensation or to have your old number reinstated will depend on the circumstances in which the number was taken away from you (so you likely have no right of redress if you had ceased paying your bills for any significant period), but the ombudsman can assess that in a very cost effective way and without you needing to pay for legal representation.
I trust the above assists.
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