Hi @RK19,
I'm so sorry to hear about this. It sounds like a fake dealer fraud. The caller will have been impersonating Three, and the DPD courier will most likely be one of the fraudsters too.
There's a few variations on this, but they essentially mimic an upgrade or new contract sales call, then using the info obtained from you on the fake call, submit an upgrade, but will choose different devices from what was discussed. This sets precedent for "returning" the phone and any accessories, but the fraudsters then intercept this by either posing as a courier, or asking the customer to post the package back to an address they can collect from.
It sounds like customer services misunderstood the situation, as they would normally raise a fraud investigation for you. I'd recommend to use the link supplied by @Officina-red for more info on how to raise the fraud with us, or as you've mentioned autism and ADHD, you're also entitled to contact our Customer Wellness Team for additional support. They're specialists in accessibility and vulnerability so I'm sure they'll be able to help.
Thanks,
Jonathan
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