If the debt has been passed to a debt collection agency then it's them you would need to contact and pay.
If the debt is still with Vodafone UK then you can also ring 03333040191 and arrange to pay or use a Vodafone payg sim to ring 191 or use someone's phone you know who's on Vodafone and ring 191 from their phone.
Please be aware that Vodafone won't agree to remove a default on your Credit File which lasts 6 years if the default is warranted, which sounds like it possibly is warranted.
I wish you all the best with this.
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Samsung Galaxy s²³ Ultra 512gb Phantom Black.