Is my refurbished iPhone locked to network

Hello. I bought a replacement refurbished iPhone off Apple after my vodaphone iPhone got water damaged. I have now sold the refurbished phone. Before selling it I put an orange SIM card in it and no errors came up and it appeared to work fine. The buyer is now saying he is unable to use the phone as it is network locked. Can anyone tell me if refurbished phones bought off Apple are network locked or unlocked please?

Thanks!