Pay monthly phone upgrade says £0 upfront but checkout wants a deposit

Hi all, I'm trying to upgrade my phone contract and the advert says £0 upfront. When I get to checkout it's asking for a £75 deposit and a security check. Is that normal? I’ve been with Nexora about 8 months and never missed a payment. Just feels a bit odd, and now I’m worried it’ll affect my credit if I keep going. Anyone else had this?